Fsu Otec Ppt

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


This shows that angled fins create more turbulence. Increases the temperature of the flow which is coming off these fins


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.


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.


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

Fsu Otec Ppt

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