CHAPTER 1 INTRODUCTION - Indian Institute of Science€¦ · Finally the heat is removed from tube surface by convection to air flowing through it Air-side resistance to heat-transfer

Investigation on Heat transfer Characteristics of Dimpled & Flat surface

Department of Mechanical Engg, N.M.I.T PAGE 1

CHAPTER 1

INTRODUCTION

1.1 INTRODUCTION:

Extensive Research has been focused on utilization of non-renewable energyimproving the

efficiency of universal process of heat exchange is one such area whichattracts a lot ofattention.

Escalating efficiency of heat transfer is useful in variety ofapplications such asmicro and macro

scale gas turbine internal airfoilcooling, heat exchangers, fuel components ofnuclear power

plants, electroniccooling combustion chambersand liners,powerful semiconductor devices, bio

medical devices, etc. compact heat exchangers and gas turbine internal air foil cooling are two

applications which has been the subject ofstudy for a number of researchers over the recent

years.

Compact heat exchangers are used immensely in trucking Industry's radiators to reduce the

excessive thermal energy. Improved efficiency of compact heat exchangers can permit radiators

to leading to smaller frontal area and thus can lead to substantial fuel saving in compact heat

exchanger there are three aspects of heat transfer. The main aspect is the convection of heat from

fluid to the walls of heat exchanger. The heat is further conducted through the walls of the tube

Finally the heat is removed from tube surface by convection to air flowing through it Air-side

resistance to heat-transfer in samall heat exchangers has between 70-80 % of the total resistance

and hence any improvement in the efficiency of a compact heat exchanger is focused on

increasing the air side convective heat-transfer.

Heat Exchangers are widely used in various thermal power plants, means of transport heating

and, electronic equipment‟s in space vehicles, air conditioning systems in all these applications,

improvements in the efficiency of heat exchangers can lead to valuable cost,material savings&

space Therefore, considerable research has been done in past to attempt to find effective ways to

increase the efficiency of heat exchangers.

The study of improved heat transfer performance is referred to as the heat transfer enhancement,

improvement. In general, it means an increase in heat transfer coefficient. Increasing heat

transfer is of special interest in channel flow where the rate of heat transfer between fluid and



channel wall decays as boundary layers grows on channel walls & flow tends to be become fully

developed increasing techniques can be classified as PASSIVE & ACTIVE methods. For both

two-phase single phase and heat transfer effective heat transfer techniques has been reported.

1.2 Active Techniques:

Active techniques don‟t require direct input of external power but rather they use it from

system itself which leads in increase in pressure drop. They use generally use surface or

geometrical modifications to flow the channel by incorporating inserts or additional devices.

Extended surface:

The extended surfaces provide effective heat transfer enlargement. The new

developments had led to modified finned surfaces that also trend to improve the heat

transfer coefficient by changing the flow field in addition to increase in the area.

Rough surface:

Rough surface has surface modifications that promote turbulence in flow field in the

wall region, primarily in phase flows, without increase in the heat-transfer in the

surface area.

Coiled tubes:

The coiled surface lead to relatively more compact heat-exchangers. As it produces

secondary flows & vortices which further promote higher heat transfer coefficients in

single phase flows as well as in most regions of boiling. And reduces the

Hydrodynamic resistance for fluid flow over surface.

Dimpled surface:

These surfaces are used as heat transfer augmentation instead of protruding the area

in flow stream, concavities or impressions are imprinted inside the surface.



Swirl flow devices:

Swirl flow devices produce and superimpose swirl slow or secondary recirculation

on the axial flow in the channel. And these include helical strip or cored screw,

twisted tapes, type tube inserts. They can be used for two phase flows and single

flow phase.

1.3 Passive techniques:

In passive techniques external power is used for facilitating the desired flow modifications

with increase in heat transfer rate.

Fluid vibration:

Its primarily used in single phase flows & considered as most practical type of

vibration enhancing technique.

Suction:

Suction involves either vapor removal through the porous heated surface in nucleate

or film boiling, or fluid withdrawal through porous heating surface in single phase

flows.

Jet impingement:

This involves direction of heating or cooling fluid perpendicularly and obliquely to

the heat-transfer surface.

Mechanical aids:

Instruments stir the fluid by mechanical means & by rotating surface. These include

scrapped surface heat and mass exchangers and rotating tube heat exchangers.

Surface vibration:



Surface vibration are applied in single phase flows to obtain higher heat-transfer

coefficients.

1.4 FLOW MECHANISM OVER THE FLAT PLATE:

When the particles of uniform stream of fluid approaches the flat plate closest to the plate

encounter skin friction and slow down. As they apply retarding shear force above the

layer immediately which slows them down and in turn slows down the layer above it and

so on. As it can be seen in the figure this results in the velocity gradient normal to the

plate. At any given point along the flat plate surface if one moves up in the Y-direction,

then the effects of viscus retardation are seen to diminish until at a point the velocity is

almost equal to that of the free stream. The thick black line bounding the fluid in the

MBL denotes the extent to where the affects of skin friction from the plate, transmitted

by viscosity into the flow are felt. Normally it is taken to be the point where, the velocity

in boundary layer is 99% of the velocity of free stream.

The boundary layer thickens as the flow progresses down the flat plate. As the boundary

layer is thickening a greater proportion of higher velocity of fluid in it, the result is

inevitable. The inertial forces overcome the viscus forces and the laminar flow transitions

into turbulence resulting in well-known turbulent boundary layer.

Fig.1.1 Flow over flat plate.



1.5 VORTEX HEAT TRANSFER IMPROVING TECHNIQUE:

Fig.1.2 V.H.T.E Mechanism.

The vortex formed inside dimple causes rubbing action of the fluid flowing inside the dimple

shown in figure. Vortex heat transfer improvement usually known as VHTE is the enhancement

of heat transfer by the system of 3-D surface cavities called as dimples having dimensions,

mutual orientation & specific geometry. Every dimple act as the “Vortex Generator” which

provides a stable & intensive heat and mass transfer between the dimpled surface and gaseous

heating/ cooling media.

1.6 FLOW MECHANISM OVER A DIMPLED SURFACE:

Dimpled surfaces are commonly known for their drag reduction characteristics in external

flow over the bodies. This is because dimples cause a change in the critical Reynolds

number figure1 shows that for flow velocity profile on the vehicle central line plain near the

roof end. This leads to downstream pressure rise, which generates reverse force acting

against the main flow and generates reverse flow at downstream point-C. No reverse flow at

point-A located for the upstream of point-C because the momentum of boundary layer is

overcoming pressure gradient between point-A & point-C, there is a separation point-B,



where the pressure gradients and the momentum of the boundary layer are equibalanced. In

the lower zone close to vehicle‟s zone within a boundary layer, the air flow quickly loses its

momentum as it moves downstream due to velocity of air which in results in reversal of the

air flow.

The primary purpose of adding vortex generator is to supply the momentum from higher

region where the air flow has larger momentum compared with the lower region where it has

small momentum value. It is possible due to the streamwise vortices generated from

vortexgenerators located just before the separation point as shown in Figure2. This allows

theseparation point to shift further downstream. Shifting the separation point downstream

makes the expanded air flow to persist proportionately longer, the flow velocity at the

separation point to become slower, & consequently the static pressure to become higher. The

static pressure at a separation point governs the overall pressures in the entire flow

separation region. It works to reduce drag by increasing back pressure. Shifting the

separation point downstream. therefore, provides dual advantages in drag reduction: one is

to narrow the separation region in which low pressure constitutes the cause of drag, another

is to raise the pressure of the flow separation region. A combination of these two effects

reduces the drag acting on the vehicle. Delaying the flow separation and the drag by itself.

The effect of delaying flow separation point, however, saturates at a certain level, which

suggests that there must be an optimum size for VGs. Thus, the purpose of using VGs is to

control flow separation at the roof end of a sedan, it is so like the purpose of using VGs on

aircraft.



Fig 1.3 Schematics of vortex generator Fig.1.4 flow velocityprofile

Delaying a flow separation & the drag by itself. The effect of delaying flow separation point, and

however saturates certain level, which suggests that there must be an optimum size of a sedan, it

is so like the purpose of using VGs on aircraft.

1.7 CONVECTION PARAMETERS:

1. NUSSELT NUMBER :

It is the ratio convective to conductive heat transfer across the boundary area.

Nu = hδ/k = qconvection/qconduction

2. REYNOLDS NUMBER:

It is defined as the ratio of inertial forces to viscus forces and is a convenient parameter

for predicting for flow condition will laminar or turbulent.

Red = ρVd / µ

3. PRANDTL NUMBER:

It is the ratio of momentum diffusivity to thermal diffusivity and can be expressed as

Pr = µCp / K

The Prandtl number is the dimensionless numbers and is often used in heat transfer for

free and forced convection calculations.

4. CO-EFFICIENT OF HEAT TRANSFER:



Heat transfer co-efficient of convective heat transfer between a fluid medium and the

surface flowed over the surface.

CHAPTER 2

LITERATURE SURVEY

NatVorayos ,NopparatKatkhaw, TanongkiatKiatsiriroat,AtipoangNuntaphan [1] In

the present study, heat transfer analysis of dimpled surfaces of external flow was

investigated. A total of 14 types of dimpled surfaces are studied. The effect of dimple

pitch was examined. The experiments were carried out with airstream flows over the

heated surface with dimples. The temperature and velocity of airstream and

temperature of Dimpled surfaces were measured. The heat transfer of dimpled

surfaces was investigated and compared with the result of smooth surface. For the

staggered arrangement, the computed results show that the maximum Nusselt

number for dimpled surfaces are about 26% better than smooth surface. And for the

inline arrangement, the results show that the maximum Nusselt number for dimples

surfaces are about 25% better than smooth surface. & 2016 The Authors. Published

by Elsevier Ltd. This is an open access article under the CC.

A.I. Leontiev , N.A. Kiselev , S.A. Burtsev , M.M. Strongin , Yu. A. Vinogradov[2]

The results of an experimental investigation of the heat transfer and the hydraulic

dragin air flow past models with different configurations of vortex reliefs in the form



of spherical dimples in a plane surface are considered. To increase the reliability of

the results and to reduce their uncertainty the experiments were performed on two

aligned models, one of which was smooth (reference model), while the other was

coated with the relief under study. Both the thermal and the hydraulic parameters of

the two surfaces were simultaneously recorded. The drag coefficient was determined

by directly weighing the models under study in the form of floating elements using a

one-component strain-gauge balance. The heat transfer coefficient was determined

by recording the unsteady heat transfer process and solving the time-dependent three-

dimensional heat conduction equation using the measured temperature fields on the

surfaces under study. The two-dimensional fields of the heat transfer coefficients on

the model surfaces were obtained and the flow over the dimpled surfaces was

visualized. The Reynolds numbers dependences of the drag, heat transfer, and heat-

hydraulic efficiency were determined, Re being based on the boundary layer length.

The dependences of the mean drag (cx/cx0) and heat transfer (St/St0) coefficients on

the dimple arrangement density (streamwise and spanwise pitches) were obtained.

For the given set of parameters a heat-hydraulic optimum geometry is determined;

for this geometry the mean Reynolds analogy factor RAF = 1.1 at St/St0 = 1.21 and

cx/cx0 = 1.1.

Sumanta Acharya, Fuguo Zhou, [3] Mass/heat transfer measurements are made

using the naphthalene sublimation method in a square internal passage where one

wall has a single dimple. Four types of dimple shapes are studied: square, triangular,

circular, and teardrop. Sherwood numbers are obtained both inside and around the

dimples. Measurements are made at a Reynolds number of 21,000. In addition,

computations are performed for the same dimple geometries, and with the same flow

conditions as in the experiments. Flow patterns for the fourdimples are identified and

heat transfer distributions for each dimple are obtained.The computational results are

compared with the experimental data and showsatisfactory agreement. Both the

experimental and numerical results suggest thatthe teardrop dimple has the highest

heat /mass transfer among the four dimple shapesstudied. [DOI: 10.1115/1.4006315]



Mr. Pardeep Singh & Harvinder Lal, [4] In this research, the heat transfer

performance of fin is analysed by design of fin with various extensions such as

rectangular, circular, trapezium and triangular extensions.The heat transfer

performance of fins with same geometry having various extensions and without

extensions are compared.Near about ranging 5% to 13% more heat transfer can be

achieved with these various extensions.

As the surface area increase due to extensions, there is more fluid contact and hence

there is more rate of heat transfer.On comparison among various shapes of

extensions, rectangular extensions provide greatest heat transfer.

Gaurang Sharma &AkshayPanchaity, [5] A fin is an extended surface from an object

to increase the rate of heat transfer.Extensions on finned surfaces are used to increase

the surface area of the fin in contact with the fluid flowing around it.Even by

increasing the heat transfer coefficient h, we can increase the heat transfer. But we

have to install pump or fan,or replacing the existing one with large one. Soits not an

economical process.It concludes by getting a results as, Rectangular fins have higher

heat transfer rate.

Mr.Bhushan S. Rane, [6] Heat sinks with fins are generally used to enhance the heat

transfer rate in many industrial applications such as cooling of electronic, power

electronic and automotive components.Due to more heat generation in theobjects,

development of effective efficient fin heat sink is required.Here we enhance the heat

transfer rate by providing proper interruptions, such as staggered interruptions.Proper

selection of the interruption length increases the heat transfer rate.

Mr.Harper and Brown, [7] Isothermal fin may be considered as a fin of infinite

thermal conductivity.Fin temperature distribution and fin efficiency are expressed in

terms of hyperbolic, power law and Bessel‟s functions depending upon the surface



profile.Effectiveness of fin is defined as the ratio of actual heat transfer from the fin

surface to that from the truebase surface area.

Puja Waghmare, [8] did a numerical investigation of heat transfer enhancement of

flow over the bumps in circular pipe. Laminar flow model was used for CFD analysis

keeping the aspect ratio of bump as 0.33. Plain tube aluminum of diameter 22mm

(OD) with 19mm (ID) and 500mm length was used as test piece. Smooth pipe, bump

pipe & inverted bump pipe was used as specimens for research. Circular pipe with

bumps lead to greater heat transfer enhancement. More fluid mixing and boundary

layer separation occurred in circular pipe. Bump pipe was more effective than

inverted bump pipe. Experimentally, the bump pipe gave heat transfer co-efficient &

Nusselt number about 30-40% more than inverted bump pipe. Due to CFD analysis

results, the bump pipe gives heat transfer co-efficient & Nusselt number about 40-

50% more than the inverted bump pipe. There was about 6-9% deviation in between

experimental result & CFD analytical results.

Avinash A Ranavare, [9] conducted an experimental analysis of heat transfer

enhancement over dimpled surface on one side of plate. Aluminum plates of

dimensions 400*72*6 mm^3 used as test surfaces. Dimpled plate with spherical

inline pattern (204 dimples) & 192 dimples of conical inline pattern were on top

surface. Diameter & depth of dimple were 6mm & 3mm respectively. Reynolds

number based on channel hydraulic diameter was varied from 2000-8500. The study

concluded as more heat transfer enhancement on dimpled surface with lesser

pressure drop penalty. Heat transfer rate from test surface increased with increase in

Reynolds number of flowing fluid at all Reynolds number considered Nusselt

number augmentation increased as dimple density of test surfaces increased.

Maximum nusselt number was obtained for staggered arrangement than inline

arrangement of dimples keeping other parameters constant.

NopparatKathkaw, [10] investigated the heat transfer behavior of flat plate having

45° ellipsoidal dimpled surfaces. 10 type of dimple arrangements and dimple



intervals are studied. Velocity of airstream was varied from 1-5 m/s. Heat transfer of

dimpled surfaces were determined and compared with the result of smooth surface.

The air side heat transfer performance is augmented approximately 10-22% at all

Reynolds number & all dimple arrangements. For staggered arrangements, the

dimple pitch of SL/D minor = 1.875 and ST/D minor = 1.875 yielded optimum

thermal resistance values of about 21.7% better than flat plate. For inline

arrangements, the dimple pitch of SL/D minor = 1.875 and ST/D minor = 3.125

yielded optimum thermal resistance values of about 15.8% better than flat plate. Co-

relations of present experiment in both staggered and inline arrangement were

obtained.

CHAPTER 3

OBJECTIVE

• Creating 3D models in CATIA design tool of flat and dimpled surfaces.

•Conducting CFD analysis on the designed models using ANSYS workbench.

•Fabrication of CATIA design models using CNC machine.

•Assembly of the workpiece for experimentation.

•Conduction of experiments on the workpieces

•Finding results and interpretation of the results of the workpieces

•To know and conclude the highest heat transfer performance and highest pressure drop of the

workpieces.



CHAPTER 4

EXPERIMENTAL SETUP

4.1 A SCHEMATIC DIAGRAM OF THE EXPERIMENTAL SET UP IS

SHOWN BELOW :



Fig : 4.1.1 : specimen inside the duct

Fig 4.1.2: experimental set up

4.2 COMPONENT SPECIFICATIONS

The components used in experiment:

a) Heater coil

b) Infrared temperature gun

c) Digital anemometer

d) Asbestos (insulator)

e) Manometer

f) Specimen

4.2.1 Heater coil

The heater coil used in this experiment is flat plate MICA heater with the following

specifications

Heater body : Zintec coated steel

Size : 150 * 75 * 4 mm^3

Connection cable : copper wire exiting at one end



Cable length : 1000 mm

Supply voltage : 230 V AC

WATT density : 750 – 1000 W

Temperature (max) : 200 C

Fig 4.2.1.1: heating coil

4.2.2: Infrared temperature gun

A precision infrared temperature sensor has been used in the experimental set

up(AR360A+).

Features :

Calibrated in Celsius

Temperature measuring range -50 to 500

Can be operated at any points

Suitable for any weather conditions

Upto 30 meters + objects temperature can be measured



Fig 4.2.2.1: infrared temperature gun

4.2.3 Digital anemometer

A hand held digital anemometer has been used in our experimental set up to adjust the

velocity of blower to induce laminar flow

The anemometer measures air velocity range upto 0 -30 m/s



Fig 4.2.3.1: Digital Anemometer

Applications :

Wind speed measurement

4.2.4 Asbestos:

It acts as heat resistance and insulation to the specimen

It acts as a shock proof agent

To resist external heat flow

Fig 3.2.4.1: Asbestos



CHAPTER 5

EXPERIMENTAL METHODOLOGY

5.1 A SCHEMATIC DIAGRAM OF THE EXPERIMENTAL SET UP IS

SHOWN BELOW:

Fig 5.1.1: specimen inside the duct



Fig 5.1.2: experimental set up

5.2: MANUFACTURING OF TEST PLATES

3 – Dimensional models of our specimens were designed by the design tool CATIA

Rectangular test plates of aluminum material (“Al He9/Al 6063”) of thickness 7 mm

having dimensions 170* 120 mm^2 were sketched

2 types of shapes i.e. spherical dimple inline and spherical dimple staggered are followed

in manufacturing of plates.

Vertical milling operations was used to create bumps of diameter 9mm and depth of 4mm

in aluminum plate of thickness 7mm were carried out.

For a rectangular inline arrangement 5 rows and 8 columns were employed, thus making

it a total of 40 dimples of spherical shapes.

For rectangular staggered arrangement 5 rows and 8 columns were employed, thus

making it a total of 40 dimples of spherical shapes.

Test section co ordinate system are employed for the study. Y co ordinate is normal to the

test surface and X co ordinate is along air flow direction.

A total of 3 test piece are used in

analysis of heat transfer from each plate and compare their 1 thermal performance.



5.3 DESIGN OF PLATES

5.3.1 : FLAT PLATE :

Fig 5.3.1.1 : CATIA model ( 170 mm x 120 mm x 7mm )



Fig 4.2.1.2 Flat plate

5.3.2 : PLATE WITH SPHERICAL DIMPLED INLINE ARRANGEMENT



Fig 5.3.2.1 CATIA model

Fig 5.3.2.2 : machined model

5.3.3 : PLATE WITH SPHERICAL DIMPLED STAGGERED ARRANGEMENT



Fig 5.3.3.1: CATIA model

Fig 5.3.3.2: machined model

5.4: ASSEMBLY PROCEDURE:



A flat plate of similar dimensions of the specimen has been gas welded with aluminum

pipes of 0.5inch diameter on either sides.

A heating coil was soldered using copper wires and was passed through the aluminum

pipes.

A mica sheet was placed under heating coil wires to prevent electric shocks.

The heating coil was sandwiched between specimen and aluminum welded flat plate.

The aluminum welded flat plate was covered with asbestos sheet to prevent heat losses.

For experimentation, we have used a apparatus where it contained a duct with a blower

where the was passed at desirable velocity.

The velocity was regulated by a knob.

The heat flux was varied by dimmer stat.

Further the blower was connected to manometer, where we obtained manometer head.

5.5 :TESTING PROCEDURE :

The assembled set up is put inside the duct for a channel for proper forced convection.

The plug is connected to the dimmer stat.

The power supply is turned on, the temperature is adjusted by velocity knob and current

knob.

The specimen reaches its steady in 5-10 minutes.

After it reaches the steady state, the blower is turned on and after two minutes of wait, the

temperature at each dimple is noted using the infrared temperature gun.

The velocity of air flow is noted using anemometer.

After noting down the temperatures the velocities are increased and the trails is

conducted for 5 different velocities keeping the heat supply constant.

The pressure head is calculated using the difference in manometer.

CHAPTER 6



CALCULATIONS

6.1 FORMULAE:

Volume flow rate of air through the duct -

Q = AxV m3/sec

Where, A = area of the duct in m2.

V = velocity in m/sec.

Film temperature -

Tf = (Ts + T∞) / 2

Where, Ts = (T1+T2+T3+T4+T5+T6+T7+T8)/8

Ts = Surface temperature in °C.

Reynolds number –

Re = LV / ν

Where, L = length of the plate in „m‟.

V = velocity in m/sec.

ν = kinematic viscosity in m2/sec.

Average Nusselt number –

Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Where, Re = Reynolds number.

Pr = Prandtl number.

Convective heat transfer –

h = (Nu x K) / L in W/m2°k

where, K = thermal conductivity in W/m°k.

Rate of heat transfer –



q = h x A (Ts – Tf ) in W

Friction factor –

f = (2 x Δp) / [ ( L / D x h ) ρ V2

]

where, Δp = difference in pressure head in „m‟

6.2 Calculations for flat plate –

I. For velocity ‘V’ = 3 m/sec

1. Volume flow rate of air through the duct –

Q = A x V m3/sec

Area of the duct, A = 0.15 x 0.1

A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec

2. Film temperature –

Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5)/5

Ts = (80 + 77 + 77.5 + 78 + 76) / 5

Ts = 77.7 °c.

Tf = (77.7 + 27) / 2

Tf = 52.35 °c.

3. Properties of air @ Film temperature, Tf = 52.35 °C



Absolute viscosity, µ = 19.72 x 10-6

N-s/m2

Kinematic viscosity,ν = 18.18 x 10-6

m2/s

Prandtl number, Pr = 0.697

Density, ρ = 1.085 kg/m3

Thermal conductivity, K = 0.02842 w/m°k

4. Reynolds number –

Re = LV / ν

Re = (0.17 x 3) / 18.18 x 10-6

Re = 28052.80

5. Average Nusselt number –

Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu= 0.664 x 28052.801/2

x 0.6971/3

Nu =98.605

6. Convective heat transfer –


h = (98.605 x 0.02842) / 0.17

h = 16.48W/m2°k

7. Rate of heat transfer –


q = 16.48 x 0.17 x 0.12 ( 77.7 – 52.35 )

q = 8.52 W

II. For velocity ‘V’ = 3.5 m/sec




Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec


Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5)/ 5

Ts = (80+76+74+75+74.5) / 5

Ts = 75.9 °C

Tf = (75.9 + 27) / 2

Tf = 51.45 °C



N-s/m2


m2/s





Re = LV / ν

Re = (0.17 x 3.5) / 18.09 x 10-6

Re = 32891.10




Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu= 0.664 x 32891.101/2

x 0.6971/3

Nu =106.77



h = (106.77 x 0.02836) / 0.17

h = 17.81W/m2°k



q = 17.81 x 0.17 x 0.12 ( 75.9 – 51.45 )

q = 8.883 W

III. For velocity ‘V’ = 4 m/sec


Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec


Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5)/ 5

Ts = (78+76+76+74+73) / 5

Ts = 75.4 °C

Tf = (75.4 + 27) / 2



Tf = 51.2 °C



N-s/m2


m2/s





Re = LV / ν

Re = (0.17 x 4) / 18.072 x 10-6

Re = 37627.26


Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu= 0.664 x 37627.261/2

x 0.6971/3

Nu =114.19



h = (114.19 x 0.02834) / 0.17

h = 19.036W/m2°k



q = 19.036 x 0.17 x 0.12 ( 75.4 – 51.2 )

q = 9.39 W

IV. For velocity ‘V’ = 4.5 m/sec




Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec


Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5)/ 5

Ts = (78+76+75+72+71.5) / 5

Ts = 74.5 °C

Tf = (74.5 + 27) / 2

Tf = 50.75 °C



N-s/m2


m2/s





Re = LV / ν

Re = (0.17 x 4.5) / 18.026 x 10-6

Re = 42438.69


Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105



Nu = 0.664 x 42438.691/2

x 0.6971/3

Nu =121.28



h = (121.28 x 0.02831) / 0.17

h = 20.19W/m2°k



q = 20.19 x 0.17 x 0.12 (74.5 – 50.75)

q = 9.78 W

V. For velocity ‘V’ = 5 m/sec


Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec


Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5)/ 5

Ts = (77+75.5+74+73+71) / 5

Ts = 74.1 °C

Tf = (74.1 + 27) / 2

Tf = 50.55 °C





m2/s





Re = LV / ν

Re = (0.17 x 5) / 18.006 x 10-6

Re = 47206.48


Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu = 0.664 x 47206.481/2

x 0.6971/3

Nu =127.91



h = (127.91 x 0.02829) / 0.17

h = 21.28W/m2°k



q = 21.28 x 0.17 x 0.12 (74.1 – 50.55)

q = 10.22 W

6.3Calculations for dimpled in-line aluminum plate –



I. For velocity ‘V’ = 3 m/sec


Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec


Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5+T6+T7+T8)/ 8

Ts = (62.8 + 74.13 + 75.4 + 80 + 68.8 + 55.16 + 60.23 + 60.43) / 8

Ts = 67.11 °c.

Tf = (67.11 + 27) / 2

Tf = 47.05 °c.



N-s/m2


m2/s





Re = LV / ν

Re = (0.17 x 3) / 17.65 x 10-6



Re = 28895.18


Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu= 0.664 x 28895.181/2

x 0.6981/3

Nu =100.12



h = (100.12 x 0.02805) / 0.17

h = 16.52W/m2°k



q = 16.52 x 0.17 x 0.12 ( 67.11 – 47.05 )

q = 6.76 W

8. Friction factor –

f = (2 x Δp) / [ ( L / D x h ) ρ V2

]

f = (2 x 2 ) / [ ( 0.17 / 0.001 x 16.52 ) 1.17 x32

]

f = 0.276

II. For velocity ‘V’ = 3.5 m/sec –


Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec




Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5+T6+T7+T8)/ 8

Ts = (82.8 + 94 + 98.46 + 97.23 + 91.6 + 59.93 + 67.26 + 60) / 8

Ts = 81.41 °c.

Tf = (81.41 + 27) / 2

Tf = 54.20 °c.



N-s/m2


m2/s





Re = LV / ν

Re = (0.17 x 3.5) / 18.37 x 10-6

Re = 32389.76


Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu= 0.664 x 32389.761/2

x 0.6971/3

Nu =105.95



h = (105.95 x 0.02855) / 0.17



h = 17.79W/m2°k



q = 17.79 x 0.17 x 0.12 ( 81.41 – 54.20 )

q = 9.87W


f = (2 x Δp) / [ ( L / D x h ) ρ V2

]

f = (2 x 2 ) / [ ( 0.17 / 0.001 x 17.79 ) 1.17 x 3.52 ]

f = 0.292

III. For velocity ‘V’ = 4 m/sec –


Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec


Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5+T6+T7+T8)/ 8

Ts = (85.46 + 73.86 + 80.73 + 85.16 + 89.53 + 66.67 + 73.9 + 71.53) / 8

Ts = 78.35 °c.

Tf = (78.35 + 27) / 2

Tf = 52.67 °c.





N-s/m2


m2/s





Re = LV / ν

Re = (0.17 x 4) / 18.22 x 10-6

Re = 37321.62


Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu= 0.664 x 37321.621/2

x 0.6971/3

Nu =113.73



h = (113.73 x 0.02844) / 0.17

h = 19.02W/m2°k



q = 19.02 x 0.17 x 0.12 ( 78.35 – 52.67 )

q = 9.964 W


f = (2 x Δp) / [ ( L / D x h ) ρ V2

]

f = (2 x 2 ) / [ ( 0.17 / 0.001 x 19.02 ) 1.17 x 42 ]

f = 0.298



IV. For velocity ‘V’ = 4.5 m/sec –


Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec


Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5+T6+T7+T8)/ 8

Ts = (82.73 + 73.86 + 90.76 + 101.03 + 102.9 + 74.4 + 76.4 + 80.23) / 8

Ts = 85.28 °c.

Tf = (85.28 + 27) / 2

Tf = 56.14 °c.



N-s/m2


m2/s





Re = LV / ν

Re = (0.17 x 4.5) / 18.57 x 10-6

Re = 41195.47




Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu= 0.664 x 41195.471/2

x 0.6961/3

Nu =119.43



h = (119.43 x 0.02868) / 0.17

h = 20.14W/m2°k



q = 20.14 x 0.17 x 0.12 ( 85.28 – 56.14 )

q = 11.97W


f = (2 x Δp) / [ ( L / D x h ) ρ V2

]

f = (2 x 3 ) / [ ( 0.17 / 0.001 x 20.14 ) 1.17 x 4.52 ]

f = 0.300

V. For velocity ‘V’ = 5 m/sec –


Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec




Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5+T6+T7+T8)/ 8

Ts = (92.66 + 89.66 + 97.13 + 98.7 + 101.6 + 66.46 + 74.7 + 69.73) / 8

Ts = 86.33 °c.

Tf = (86.33 + 27) / 2

Tf = 56.66 °c.



N-s/m2


m2/s





Re = LV / ν

Re = (0.17 x 5) / 18.62 x 10-6

Re = 45649.83


Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu= 0.664 x 45649.831/2

x 0.6961/3

Nu =125.72



h = (125.72 x 0.02872) / 0.17

h = 21.24W/m2°k





q = 12.85 x 0.17 x 0.12 ( 86.33– 56.66 )

q = 12.85W


f = (2 x Δp) / [ ( L / D x h ) ρ V2

]

f = (2 x 3 ) / [ ( 0.17 / 0.001 x 21.24 ) 1.17 x 52 ]

f = 0.299

6.4 Calculations for dimpled staggered aluminum plate –

I. For velocity ‘V’ = 3 m/sec –


Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec


Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5+T6+T7+T8)/ 8

Ts = (72.3 + 80.1 + 76.36 + 75.4 + 68.76 + 79.13 + 65.53 + 55.36 ) / 8

Ts = 71.61 °c.

Tf = (71.61 + 27) / 2



Tf = 49.30 °c.



N-s/m2


m2/s





Re = LV / ν

Re = (0.17 x 3) / 17.88 x 10-6

Re = 28523.48


Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu= 0.664 x 28523.481/2

x 0.6981/3

Nu =99.476



h = (99.476 x 0.02821) / 0.17

h = 16.50W/m2°k



q = 16.50 x 0.17 x 0.12 ( 71.61 – 49.30 )

q = 7.50W


f = (2 x Δp) / [ ( L / D x h ) ρ V2

]



f = (2 x 1.5 ) / [ ( 0.17 / 0.001 x 16.50 ) 1.17 x 32 ]

f = 0.286

2 For velocity ‘V’ = 3.5 m/sec –


Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec


Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5+T6+T7+T8)/ 8

Ts = (85.63 + 83.3 + 79.7 + 81.9 + 82.23 + 52.76 + 71.57 + 73.48) / 8

Ts = 76.31 °c.

Tf = (76.31 + 27) / 2

Tf = 51.65 °c.



N-s/m2


m2/s





Re = LV / ν



Re = (0.17 x 3.5) / 18.11 x 10-6

Re = 32854.77


Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu= 0.664 x 32854.771/2

x 0.6971/3

Nu =106.71



h = (106.71 x 0.02837) / 0.17

h = 17.80W/m2°k



q = 17.80 x 0.17 x 0.12 ( 76.31 – 51.56 )

q = 8.95W


f = (2 x Δp) / [ ( L / D x h ) ρ V2

]

f = (2 x 2 ) / [ ( 0.17 / 0.001 x 17.80 ) 1.17 x 3.52 ]

f = 0.298

3 For velocity ‘V’ = 4 m/sec –


Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec




Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5+T6+T7+T8)/ 8

Ts = (92.3 + 90.46 + 83.6 + 80.6 + 83.7 + 78.53 + 86.93 + 79.73) / 8

Ts = 84.48 °c.

Tf = (84.48 + 27) / 2

Tf = 55.74 °c.



N-s/m2


m2/s





Re = LV / ν

Re = (0.17 x 4) / 18.53 x 10-6

Re = 36697.24


Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu= 0.664 x 36697.241/2

x 0.6961/3

Nu =112.72





h = (112.72 x 0.02866) / 0.17

h = 19.004W/m2°k



q = 19.004 x 0.17 x 0.12 ( 84.48 – 55.74 )

q = 11.14W


f = (2 x Δp) / [ ( L / D x h ) ρ V2

]

f = (2 x 2.5 ) / [ ( 0.17 / 0.001 x 19.004 ) 1.17 x 42 ]

f = 0.306

4 For velocity ‘V’ = 4.5 m/sec –


Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec


Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5+T6+T7+T8)/ 8

Ts = (95.5 + 85.56 + 88.1 + 88.03 + 88 + 58.26 + 70.23 + 83.1) / 8

Ts = 82.09 °c.

Tf = (82.09 + 27) / 2

Tf = 54.54 °c.





N-s/m2


m2/s





Re = LV / ν

Re = (0.17 x 4.5) / 18.41 x 10-6

Re = 41553.50


Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu= 0.664 x 41553.501/2

x 0.6971/3

Nu =120.009



h = (120.009 x 0.02857) / 0.17

h = 20.16W/m2°k



q = 20.16 x 0.17 x 0.12 ( 82.09 – 54.54 )

q = 11.33W


f = (2 x Δp) / [ ( L / D x h ) ρ V2

]

f = (2 x 3 ) / [ ( 0.17 / 0.001 x 20.16 ) 1.17 x 4.52 ]



f = 0.308

5 For velocity ‘V’ = 5 m/sec –


Q = A x V m3/sec


A = 0.015 m2

Q = 0.015 x 3

Q = 0.045 m3/sec


Tf = (Ts + T∞) / 2

Ts = (T1+T2+T3+T4+T5+T6+T7+T8)/ 8

Ts = (97.1 + 91.4 + 92.23 + 89.93 + 87.4 + 73.53 + 88.06 + 86.01 ) / 8

Ts = 88.21 °c.

Tf = (88.21 + 27) / 2

Tf = 57.60 °c.



N-s/m2


m2/s





Re = LV / ν

Re = (0.17 x 5) / 18.72 x 10-6



Re = 45405.98


Nu = 0.664 x Re1/2

x Pr1/3

for Re< 5 x 105

Nu= 0.664 x 45405.981/2

x 0.6961/3

Nu =125.38



h = (125.38 x 0.02879) / 0.17

h = 21.23W/m2°k



q = 21.23 x 0.17 x 0.12 ( 88.21 – 57.60 )

q = 13.25W


f = (2 x Δp) / [ ( L / D x h ) ρ V2

]

f = (2 x 3.5 ) / [ ( 0.17 / 0.001 x 21.23 ) 1.17 x 52 ]

f = 0.301

CHAPTER 7

RESULTS AND DISCUSSION

7.1: HEAT TRANSFER CO EFFICIENT VS REYNOLDS NUMBER :



Figure shows the effect of dimple pitch of the staggered & inline arrangement & flat plate surfaces of the air side heat transfer performance. Results are termed as Heat transfer co-

efficient vs. Reynoldsnumber.Asseeninthefigure, Heat transfer co-efficient values are augmented at all Reynolds number and all dimple pitches compared to the flat plate. The yields

of the highest heat transfer co-efficient showthatthemaximumheat transfer co-efficient for dimpled surfaces arebetterthansmoothsurface.Andfor theinlinearrangement,theresultsshow

that the maximum heat transfer co-efficient for dimples surfaces are better than smooth surface.

7.2: NUSSELT NUMBER VS REYNOLDS NUMBER :



Figure shows the effect of dimple pitch of the staggered & inline arrangement & flat plate surfaces of the air side heat transfer performance. Results are termed as Nusselt number vs.

Reynoldsnumber.Asseeninthefigure, Nusselt number values are augmented at all Reynolds number and all dimple pitches compared to the flat plate. The yields of the highest Nusselt

numbershowthatthemaximumNusselt number for dimpled surfaces

arebetterthansmoothsurface.Andfor theinlinearrangement,theresultsshow that the maximum Nusselt number for dimples surfaces are better than smooth surface.

7.3: FRICTION FACTOR VS REYNOLDS NUMBER



Figure shows the effect of dimple pitch of the staggered & inline arrangement & flat plate

surfaces of the air side heat transfer performance. Results are termed as Friction factor vs.

Reynoldsnumber.Asseeninthefigure, friction factor values are augmented at all Reynolds

number and all dimple pitches compared to the flat plate. The yields of the highest friction

factorshowthatthemaximumfriction factor for dimpled surfaces

arebetterthansmoothsurface.Andfor theinlinearrangement,theresultsshow that the maximum

Nusselt number for dimpled inline surfaces are not so better than staggered arrangement

dimple surfaces.

CHAPTER 8



CONCLUSION

The heat transfer rate from test surface increases in flowing fluid velocity and heat input.

The usage of dimples on surface results in heat transfer rising in forced convection heat

transfer with lesser pressure drop penalty.

The value of max Nusselt number obtained for staggered arrangement of dimples is greater

than the inline arrangement keeping all parameters constant. It shows that for heat transfer

enhancement, staggered arrangement is more effective than inline arrangement.

The results got from friction factor and varying velocities of Reynolds number has helped to

prove that staggered arrangement has more pressure drop.

Due to more turbulence achieved in staggered arrangement the heat transfer co efficient vs

Reynolds number graph shows that staggered arrangement of dimples has the highest heat

transfer performance compared to flat and inline arrangement.

CHAPTER 9



SCOPE FOR FUTURE WORK

In the present work, experiments were carried for limited low velocity and lower range of

Reynolds number. This could be used for higher range of Reynolds number and using larger

dimensions of test surface.

Different shapes like rectangular, triangular, almond shapes of dimples can be used instead of

spherical dimples on test surfaces.

Test plate material can be changed such as copper, which is very good conductor of heat and

performance can be compared with different material combinations.

REFERENCES



1. Puja Waghmare, prof. Ravi H.C, mechanical engineering department, DYPSOEA, Ambi

, Maharashtra ,India. Numerical investigation of heat transfer enhancement of flow over

the bumps in circular pipe. International journal for engineering applications and

technology (IJFEAT). Issue 8 volume 3, July 17.ISSN : 2321 – 8143

2. Avinash A. Ranaware ,Iftikarahmed H patel , mechanical engineering department,

SVPM-SCOE, Malegaon, Baramati, pune, India. Experimental analysis on heat transfer

enhancement over dimpled surface on one side of plate. International journal on recent

technologies in mechanical and electrical engineering (IJRMEE). volume: 4 issue 9th

sept,2017. ISSN: 234927947

3. Nat Vorayos ,NopparatKatkhaw, TanongkiatKiatsiriroat ,AtipoangNuntaphan [1] In the

present study, heat transfer analysis of dimpled surfaces of external flow was

investigated. A total of 14 types of dimpled surfaces are studied.

4. A.I. Leontiev , N.A. Kiselev , S.A. Burtsev , M.M. Strongin , Yu. A. Vinogradov [2] The

results of an experimental investigation of the heat transfer and the hydraulic drag in air

flow past models with different configurations of vortex reliefs in the form of spherical

dimples in a plane surface are considered.

5. Sumanta Acharya, Fuguo Zhou, [3] Mass/heat transfer measurements are made using the

naphthalene sublimation method in a square internal passage where one wall has a single

dimple. Four types of dimple shapes are studied: square, triangular, circular, and

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6. NopparatKathkaw, [10] investigated the heat transfer behavior of flat plate having 45°

ellipsoidal dimpled surfaces. 10 type of dimple arrangements and dimple intervals are

studied. Velocity of airstream was varied from 1-5 m/s.

7. Gaurang Sharma &AkshayPanchaity, [5] A fin is an extended surface from an object to

increase the rate of heat transfer. Extensions on finned surfaces are used to increase the

surface area of the fin in contact with the fluid flowing around it.



Documents

CHAPTER 1 INTRODUCTION - Indian Institute of Science€¦ · Finally the heat is removed from tube surface by convection to air flowing through it Air-side resistance to heat-transfer