FSU-OTEC Group #14 Adam Choby Simon Chong Matthew Cocking Mike Collins
Lockheed Martin and Oak Ridge National Laboratory
OTEC Working Power Cycle
Plate-Fin Heat Exchanger
Poco Foam A new graphite foam is used for the inner fins
5 times less dense than aluminum Bulk Thermal Conductivity roughly the same as
aluminum (180 W/m*K) Open Porous Thermal Diffusivity 3 times faster than copper
Evaporator SpecificationsEvaporators: Warm Seawater Flow Warm Seawater Inlet Temperature Warm Seawater Outlet Temperature Warm Seawater Pressure Drop (Desired) Ammonia Boiling Temperature Ammonia Pressure Drop Heat Exchanged from Warm Seawater to Ammonia Heat Exchanger Life (prior to major overhaul) 115 25.7 22.9 27 20 15 1280 10+ m3/sec Deg C Deg C kPa Deg C kPa MW thermal years
Heat Exchanger Optimization To optimize the final Unit Cell design, different
parameters were tested. Each tests results were compared to the last and
modified accordingly in order to have the optimal choice for each attribute.
Tests were conducted to optimize: Fin geometry Height and length of channels Fin configuration Effectiveness of holes in fins
Fin Geometry Tests Different geometries of fins had a big difference when
tested in the same environment. The geometries that were tested were: Rectangular Fins
Diamond Fins Diamond Fins (different sizes and a couple angled) Diamond Fins (different angles)
Rectangular Fins
Rectangular Fins (Temperature)
Rectangular fins showed that they do increase heat transfer. This is highest in the wake region of the fin, in areas of low pressure
Rectangular Fins (Velocity)
This is used to illustrate the way the fins create high velocity flow regions, while having very low velocity regions.
Diamond Fins
Diamond Fins (Temperature)
High temperature in region directly behind fin. Eliminated low temperature regions by reducing the wake region
Diamond Fins (Velocity)
This illustrates that there are no low velocity regions when utilizing the diamond fins. The velocity actually doubles where the fins create a channel which has one half the distance of the inlet.
Diamond Fins Test #2
Diamond Fins (Temperature)
This shows that angled fins create more turbulence. Increases the temperature of the flow which is coming off these fins
Diamond Fins (Velocity)
Note fin configurations can increase velocity at specific spots to increase heat transfer. This idea will be utilized in final design.
Diamond Fins Test #3
Test 3 has various angled fins and sizes to compare flow patterns and velocity gradients around these areas.
Diamond Fins (Temperature)
This fin design shows that having angled fins increases turbulence and forces a channel of fluid through. This channel has a higher temperature than the flow that went through the center of the fins.
Diamond Fins (Velocity)
Highest velocity occurs in areas with straight fins. Angled fins create low pressure, low velocity wake regions. More turbulence = More heat transfer
Fin Geometry It can be concluded that fins which induce turbulent
flow enhance heat transfer, thus increasing the temperature. Individual factors such as height and width of fins
need to be analyzed in order to further optimize this system.
Height Optimization
Evaporator Height Tests
Height Test15.6 15.4 Outlet Temp (Celsius) 15.2 15 14.8 14.6 14.4 14.2 14 0.2 0.3 0.4 0.5 0.6 0.7 Height (in) 400 300 Outlet Temperature 200 pressure 600 500
1000
Fin Height Optimization0.75 in
0.40 in
0.0 in
This is a front view of a single rectangular fin sitting on a heater plate. It shows the majority of the heat being conducted (24.3-16 deg) in the first 0.4 inches From 0.4 to 0.75 inches, the temperature changes from (16 -10 deg). Eliminating this excess fin length will increase the amount of heat transferred to the ammonia.
Fin Height Optimization (Side View)
Fin Height Conclusions Using a fin height of uniformly 0.35 inches high will be
optimal. Under 0.4 inches is the maximum area used for heat conduction. Fins this height will ensure that the fluid contacts the hottest area of foam at all times.
Altered Fin Geometry Tests
Plane section is midpoint of evaporator. 12.5ft from inlet
ResultsAltered Geometry Tests30 25 20 15 10 5 0 0 0.2 0.4 0.6 0.8 1 Quality of Vapor .5in x .5in
Feet from Inlet
.5in x .5in with holeDouble Parabolic .5in x .25in Double rectange .5in x .25in
Interesting to note pressure losses halfway and throughout full exchanger
Conclusions of Geometry Tests Using fins which have square edges and sharp angles
seems to be the best for heat transfer. This is also due to the amount of fluid which can contact the heater plate. Using parabolic or pathway holes did not seem to increase heat transfer enough to be feasible.
Full Length Tests Individual Tests werent conclusive without phase
change. 300in x 1 in channels were tested, with varying width. The extra long length ensures a phase change even without using fins.
Full Length Tests
No Fin
.25in x .5in x 300in
.5in x .5in x 300in
.75in x .5in x 300in
Results
Width Tests-300in Long30 25 Feet from Inlet 20 15 10 5 0 No Fin
.25inx.5in.5inx.5in .75inx.5in
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Quality of Vapor
Diamond Fins on Top and Bottom
This model proved to be successful. It consisted of diamonds coming from the top and bottom to enhance heat transfer while keeping pressure drop low. Mixing proved successful while reducing the overall length of the exchanger by over 60%.
Full Length Diamond Fin
Diamond Fin Outlet Quality
Quality= .74
Unit Cell This style of staggered diamond fins on the top and
bottom is the most efficient. Can facilitate necessary heat transfer in only 9 feet. Pressure drop of only 4500 Pa The unit cell of the heat exchanger comprises this design the width extended to 12 inches.
Unit Cell Calculations Needed: 1109 kg/s of Ammonia Density of ammonia: 610 kg/m^3 Unit Cell: Dimensions:
12 inch x 0.5 inch Inlet = 6 in^2 = 0.00387 m^2 108 inches long
Inlet Velocity: 0.55 m/s Mass Flow Rate per Unit Cell:
M.dot = (density)(Inlet Area)(Inlet Velocity) (610 kg/m^3)(0.00387 m^2)(0.55 m/s) = 1.298 kg/s per Unit Cell!
Overall System Mass Flow Rate Available construction volume of 10x10x30 ft for each
complete heat exchanger and condenser system Assuming aluminum plates of 0.25 inch thickness inbetween layers.
Future Optimization Techniques Utilizing holes within the fins to increase mixing Using various types of fins in the region of high quality
vapor. Vapor is easier to pump and can mix more without
sacrificing as much pressure Lower density Higher energy
Tests were conducted to see which of these techniques
may prove to be useful in later optimization.
Vapor Region Tests were conducted in the vapor region to reduce
pressure loss and enhance mixing.
Separated Rectangles
Pressure Loss is sufficiently higher than diamonds.
Separated Diamonds
Attempt #1~1500Pa
X=.554
Incorporated rectangular fins in liquid region and diamond fins in vapor region. Did not meet requirements, most likely due to large height. Smaller channel was implemented.
Attempt #2
Smaller channel was implemented thus requiring smaller pressure drop fins. Parabolic fins were implemented in liquid region, followed by longer diamonds (4in). Unable to mesh. Worst quality was 7.56x10^-5
Perforations It was seen in the previous slide that a hole going
straight thru a fin increased heat transfer. Hole analysis needed to be performed. Various types of holes were tested.
Perforations.5in x .5in (5x36) ~600 Pa
16.91 C
This model was used as a reference model for further testing.
Vertical Holes Radius=.125in
Depth=.25in Density= 1 per inch~800 Pa
16.89C
Increased pressured drop and decreased outlet temperature compared to no hole model.
Horizontal Holes (along x-axis)Horizontal Cylinders: r=.125 in z=.3in =1 per 4 Vertical Holes: r=.125in d=.025in =1 per 4~800Pa
16.65 C
Crossed Grid1st Cross: r=.125 z=.3 =1 per 2 =45~800Pa
16.17 C
Horizontal Holes (along y-axis) Radius .125in 36in long z=.3in~400 Pa
Yielded best results in liquid region.17.20C
Hole Conclusion Horizontal Holes along y-axis proved to be the best during liquid
region. Longitudinal holes within fins increases manufacturing costs,
while decreasing durability. Massive amounts of holes drastically increased computational
time, thus limiting its application to full scale models. Holes were tested and compared to other styles of fins.
Evaporator Conclusions Diamonds proved to be best in vapor region for
pressure drop. Using this style of fins in the vapor region could enhance heat transfer while the vapor is increasing quality. Utilizing holes will also increase the heat transfer further as well, but is hard to implement
Condenser SpecificationsCondensers: Deep Cold Seawater Flow Deep Cold Seawater Inlet Temperature Deep Cold Seawater Outlet Temperature Deep Cold Seawater Pressure Drop (Desired) Ammonia Condensing Temperature Ammonia Pressure Drop Heat Exchanged from Ammonia to Deep Cold Seawater Heat Exchanger Life (prior to major overhaul) 80 4.1 7.9 13.5 10