PRACTICUM REPORT

Heat Dissipation of Hot Wire

Name : Asti Diar Safitri NPM : 1206292414 Faculty : Faculty of Engineering Department : International Program/ Civil Engineering Practicum Code : KR-01 Practicum Week : 3

Unit Pelaksana Pendidikan Ilmu Pengetahuan Dasar (UPP-IPD)

University of Indonesia Depok

Purpose

Using hot wire as airflow speed sensor

Apparatus

1. Hot wire

2. Fan

3. Voltmeter and Ampere meter

4. Adjustable power supply

5. Camcorder

6. PC Unit with DAQ and automatic control device

Basic Theory

Single normal probe is a type of hotwire that generally used as sensor to give airflow

velocity information in axial direction only. This probe including a smooth-short-

metal wire that incorporated in the two-wire. Each probe tip is connected to a voltage

source. Electrical energy that flows in the probe will be dissipated by the wire into

heat energy. The amount of electrical energy that dissipated is proportional to the

voltage, electric current flowing in the probe, and duration of electric current flowing.

W = v i∆t .........( 1 ) When the probe is blown by air then it will change the value of resistance wire that

alter the amount of electrical current flowing. The faster the airflow, the bigger

changes of resistance value and the electric current that flows also changed. The

number of heat transfer that received by the probe is expressed by overheat ratio,

which is defined as:

 

Rw = wire resistance at operating temperature (blown by air)

Ra = wire resistance at ambient temperature (room)

airflow from fan

Hot wire probe must be calibrated to determine the equation that states the

relationship between voltage wire (wire voltage, E) with a reference velocity

(reference velocity, U) after the equation is obtained, then the information velocity in

each experiment can be evaluated using the equation. Equations derived in the form

of linear equations or polynomial equations.

In the experiment about to be done is to measure the voltage wire at ambient

temperature and measure the voltage across the wire when energized air produced by

the fan velocity. Airflow velocity by the fan will be varied through the power supplied

to the fan is 70, 110, 150 and 190 of the maximum power of 230 m / s.

Procedure

This rLab experiment can be done by clicking the rLab button at the bottom of this

site.

1. Activating webcam

2. Giving airflow with a speed of 0 m/s, by clicking the drop down option “adjust

the flow rate”

3. Turning on the fan motor by clicking the radio button on the "turn on the fan

power supply”

4. Measuring voltage and electric current in the hotwire by clicking the icon

"measure"

5. Repeating step 2 until step 4 for speeds of 70, 110, 150, 190 and 230 m/s

Assignment and Evaluation

1. Based on the data obtained, make a graph illustrating the relationship Voltage

Hotwire with time for each air velocity.

2. Based on the data processing above, make a graph illustrating the relationship

of hotwire voltage with wind flow velocity.

3. Make the wind velocity equation as a function of the hotwire voltage.

4. Based on the experiments and the data obtained, can we use Hotwire as a wind

speed measure?

5. Give the analysis of the results of this experiment.

Data Tubulations

Time   Wind  Velocity  

V-­‐HW   I-­‐HW  

1   0   2.112   53.9  2   0   2.112   54.7  3   0   2.112   55.0  4   0   2.112   53.9  5   0   2.112   54.5  6   0   2.112   55.2  7   0   2.112   53.9  8   0   2.112   54.2  9   0   2.112   55.4  10   0   2.112   53.9  1   70   2.048   55.2  2   70   2.050   57.3  3   70   2.048   54.5  4   70   2.050   54.5  5   70   2.048   57.4  6   70   2.048   55.0  7   70   2.047   54.2  

8   70   2.049   56.5  9   70   2.049   56.3  10   70   2.048   54.2  1   110   2.029   55.1  2   110   2.030   58.0  3   110   2.029   56.2  4   110   2.029   54.4  5   110   2.029   55.8  6   110   2.028   58.2  7   110   2.029   55.5  8   110   2.028   54.4  9   110   2.029   56.3  10   110   2.030   58.0  1   150   2.022   58.7  2   150   2.021   55.1  3   150   2.022   54.7  4   150   2.022   57.3  5   150   2.022   57.5  6   150   2.022   54.8  7   150   2.022   55.0  8   150   2.021   58.0  9   150   2.022   56.8  10   150   2.021   54.6  1   190   2.015   58.8  2   190   2.015   57.5  3   190   2.016   54.9  4   190   2.016   55.0  5   190   2.017   57.5  6   190   2.016   57.9  7   190   2.018   55.2  8   190   2.018   54.6  9   190   2.018   56.7  10   190   2.017   58.6  1   230   2.014   54.7  2   230   2.014   55.1  3   230   2.013   57.3  4   230   2.013   58.8  5   230   2.013   57.4  6   230   2.014   55.2  7   230   2.014   54.7  8   230   2.014   55.7  9   230   2.013   58.0  10   230   2.013   58.4  

Graph illustrating the relationship Voltage Hotwire with time for each air velocity

Graph illustrating the relationship of Voltage Hotwire with wind flow velocity

X = Wind velocity

Y = Average voltage

X Y X^2 Y^2 XY

0 2.112 0 4.4605 0

70 2.0485 4900 4.1963 143.395

110 2.029 12100 4.1168 223.19

150 2.0217 22500 4.0872 303.255

190 2.0166 36100 4.0666 383.154

230 2.0135 52900 4.0541 463.105

750 12.2413 128500 24.9815 1516.099

According to equation 1 above states that the amount of electrical energy that is

dissipated is proportional to the voltage, the electric current flowing in the probe, and

duration of electric current flowing.

W= V.I.t

Then the electrical energy is also proportional to the force and the distance.

W= F.s

While the value ratio depends on and velocity and time, so electrical energy depends

on the force, velocity, and time.

V.I.t = F.v.t

V.I = F.v

V = F v/I

y = m x + b

Explanation:

x is a given air flow rate

y is the voltage hotwire

m=

m= - 0.0004

b=

b= 2.0908

y= - 0.0004x + 2.0908

The equation that links voltage and hotwire air velocity is:

y= - 0.0004x + 2.0908

with y as hot wire voltage and x as the wind velocity.

Based on the equation above, if it’s including one of the wind velocity values,

obtained voltage value is almost the same (approach). For example, if you put the

value of wind speed of 70 m/s then obtained 2.0628 voltage values so that the relative

error obtained is small so it can be concluded that hotwire can be used as a wind

speed measure.

Analysis

Experiment

In this rlab KR01 experiment, it has a purpose to determine the hot wire as air

speed sensor. Due to changes in temperature on the hot wire, the experiment

was carried out until the temperature of the wire to be normal as before. Air

source such as a fan. In this KR01 experiment, the varied air velocity is 0 m /

s, 70 m / s, 110 m / s, 150 m / s, 190 m / s and 230 m / s . At the time of the air

velocity equals to 0, the value of the voltage and current values are read, have

not been affected by the wind velocity. Then the flow rate is set to 70 m / s

then the value of the legible voltage becomes smaller and readable current

value becomes greater. In addition the wind velocity to 230 m / s, the voltage

value that is read becomes smaller and the current value becomes larger. This

is because the wind flow affects the resistance value of hotwire that increases,

so it’s causing the current value increases and voltage value decreases which

is in accordance with the formula:

V = I x R

R = V / I

In addition, due to the brunt of the wind that was given force F with velocity V

in 10 seconds that affect the value resistansis, thus also affecting the value of

the transferred heat.

Result

Based on the experiment that have been conducted on KR01, showed that the

greater the speed of air that blown by fan, the smaller voltage on the wire and

the current flowing in the hotwire increases. The equations from the

calculations above, obtaining appropriate voltage value (approach) if it’s

including wind velocity value as x in this following equation:

y= - 0.0004x + 2.0908

For example, if you put the value of wind speed of 70 m/s then obtained

2.0628 voltage values. Obtained errors are relatively small for the experiment

because it’s performed with computerized thereby preventing Human Error.

Graph

Based on the graph of voltage vs velocity above, it indicates that the air

velocity is inversely proportional to the voltage generated hotwire. This is in

accordance with the procedures that the higher air velocity indicates the

smaller value of voltage and the gradient is minus. And on the other graph

which is a graph of voltage vs time shows the consistency of data despite very

small changes. This occurs because the velocity of the wind coming from the

fan does not go directly to the desired speed, the velocity accelerates before

reaching the desired speed. For example to get to the speed of 110 m / s are

initially 0 m / s, not directly from 0 m / s and then directly to 110 m / s but it

accelerates to 110 m / s first. Therefore, we measured up to 10 seconds. While

the voltage vs time graph fluctuates when probe blown by winds with certain

velocity so that the voltage values varie.

Conclutions

1. The  amount  of  electrical  energy  dissipated  is  proportional  to  the  voltage,  

the   electric   current   flows   and   the   length   of   time   the   electric   current  

flowing.

2. Wind  flow  velocity  resulting   in  a  measured  current  value  becomes   large  

(comparable)  with  the  addition  of  air  velocity  source.

3. Wind  flow  velocity  that  hits  the  hotwire  resulting  voltage  value  becomes  

small  (inversely)  with  the  addition  of  air  velocity  source.

4. The  wire   can  be  used  as   a  hotwire   air   velocity   sensor,   although   slightly  

different  results  obtained  by  calculation.

References

1. Giancoli, D.C.; Physics for Scientists & Engeeners, Third Edition,

PrenticeHall, NJ, 2000.2.

2. Halliday, Resnick, Walker; Fundamentals of Physics, 7th Edition,

ExtendedEdition, John Wiley & Sons, Inc., NJ, 2005