SOLUTION FOR HEATRANSFER

IN HYPERLOOP POD

Advanced Heat Transfer Term Project

YASH NAIK

Student Id: 108947812

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HEAT TRANSFER PROJECT – Solution for Heat transfer in Hyperloop Pod

Table of contents

• Abstract

• Introduction

• Current System

o Components

o Operation

o Calculation

• Alternate solutions

• Increasing Radiative Heat Transfer

• Increasing Convective Heat Transfer

• References

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ABSTRACT:

This report contains current method of cooling used in Hyperloop Pod with calculation and

after that some other methods to increase heat transfer rate is discussed. Focus is to enhance

radiative heat transfer as mostly it is taken for granted in system designs but can play major

role. Many Article and papers are reviewed for this as radiative heat transfer can also contribute

towards convective heat transfer also. Each method is reviewed, and conclusion is it can used

or not in pod with what changes and to what length.

INTRODUCTION:

SpaceX Hyperloop pod Competition requires each Team to Design and build a pod which is

within competition parameters and passes all preliminary as well as final design testing. So as

a member of Hyper Lynx team, representing UC Denver, I am working on designing the

Hyperloop pod for the competition. The original design intent was to avoid using a cooling

system and keep the design as simple as possible. But it is necessary for some of the

Components.

CURRENT SYSTEM:

Only few components generate heat that must be dissipated. Selected components were

researched to determine the estimated temperature rise and any cooling requirement.

The low voltage battery, Raspberry Pi, and Arduino components do not require cooling. Each

was vacuum tested to 0.14psi (7torr) at SpaceX in 2016, and no adverse heat rise was recorded.

The high voltage battery manufacturer has confirmed that cooling is unnecessary for the

batteries. The motor and motor controller both explicitly require cooling. We assume the nearvacuum

conditions provide no effective air convection.

As per the manufacturer, the EMRAX 208 liquid-cooled motor requires 7L/min of coolant flow

during operation, at a nominal pressure of 8.7psi (0.6bar). The maximum pressure is

200kPa. The listed motor efficiency is 92-98%. For heat generation calculations, a 90%

efficiency was assumed, resulting in 8kW of maximum heat generation. The motor is not likely

to spend the entire flight at maximum power, so these numbers are conservative. The Unitek

BAMOCAR-D3 motor controller lists a maximum flow of 12L/min, a maximum pressure of

130kPa. The maximum power loss (and assumed heat generation) is listed at 3kW. The

assumed time of flight will be 60sec, and this is conservative. The motor and motor controller

will generate 66kJ of heat during operation.

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The cooling system will serve as a heat sink. No radiator will be used due to the near-vacuum

conditions and short time of flight. Distilled water will be used as coolant. The system will be

sealed during very pre-flight and the reservoir tank will maintain an internal pressure of

approximately 100kPa during flight. A 1gal reservoir of distilled water exposed to 66kJ of heat

will result in a 4°C temperature rise. The system requires a pump head of 0.7m. During longterm

storage, the coolant system will be filled with a glycol mixture for anti-freezing and antimicrobial

protection.

COMPONENTS:

The motor and motor controller will have individual pumps, tubing, and sensors. This ensures

we can monitor and control coolant flow to the motor and controller independently. The pump

is a 12VDC water pump with maximum flow of 13L/min, a 4m, a maximum temperature of

60°C, and a maximum power draw of 1.2A. Each pump will receive 12VDC from the LV

power system, fused at 2.5A. Tubing is 0.375″ (9.525mm) ID nylon, rated to 450psi (3103kPa)

at 70°F (21°C) and a maximum temperature of 212°F (100°C). Fittings are High-Density

Polyethylene with a maximum pressure of 125psi (862kPa) and maximum temperature of

190°F (88°C).

The reservoir has two sensors. A pressure sensor will verify the reservoir is maintaining tank

pressure during vacuum conditions, and a temperature sensor will monitor the fluid temperature

rise during operation. The motor and motor controller will each have a pressure sensor at their

respective inlets to monitor delivered coolant pressure.

OPERATION:

Before flight and during the INIT state, the Hyper lynx crew will activate the coolant pumps

and adjust them to reach desired motor and motor controller pressures. The pumps are

controlled by variable 12VDC signal, which will be controllable through the Mission Control

interface. The pumps will continue to run for the duration of the flight, and are only deactivated

by the crew after verifying reservoir, motor, and motor controller temperatures have stabilized

to safe levels.

CALCULATIONS:

Reservoir Temperature Rise (4.1°C)

?T=(Q/M*C) (1)

?T is the temperature rise in Celsius

Q is the heat input (66kJ)

M is the mass of coolant (3.8kg)

C is the heat capacity of the coolant (4184J/kg)

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Fluid Velocity (3.68m/s)

v=Q/A (2)

v is the fluid velocity in m/s

Q is the desired flow rate (1.1667×10-4

m3

/s)

A is the cross-sectional area of the coolant tubing (3.1669×10-5

m2

)

Required Pump Pressure (76456Pa)

P1=P2+?g(z2-z1) +?v2

/2(f*L/D+?K) (3)

P1is the required pump pressure in Pa

P2 is the desired line pressure at the motor (76443Pa)

? is the density of water at 30°C (995.7kg/m3

)

g is the gravitational acceleration, (9.81m/s2

)

z2 is the elevation of the motor from the datum (0.1m)

z1 is the datum elevation of the system (0m)

v is the velocity of the coolant from Eq.1

f is the friction factor for a smooth pipe, derived from the Moody chart

D is the diameter of the tubing (0.0064m)

L is the length of tubing (0.2m)

?K is the sum of the minor loss constants

Required Pump Head (0.7m)

hp=P2-P1/?g+(z2-z1) +v

2

/2g(f*L/D+?K) (4)

hp is the pump head in m

P2is the desired line pressure at the motor (76443Pa)

P1is the required pump pressure from Eq.2

? is the density of water at 30°C (995.7kg/m3

)

g is the gravitational acceleration, (9.81m/s2

)

z2 is the elevation of the motor from the datum (0.1m)

z1 is the datum elevation of the system (0m)

v is the velocity of the coolant from Eq.1

f is the friction factor for a smooth pipe, derived from the Moody chart

D is the diameter of the tubing (0.0064m)

L is the length of tubing (0.2m)

?K is the sum of the minor loss constants

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ALTERNATE SOLUTIONS:

As for cooling system we are using is more practical and easy to implement. But it not the only

solution that can be applied to solve the problem. There are numerous ways (solution) that can

be used here but might not be practical to use them because they might be experimental or

requires lot more effort. I have found some other ways to increase the heat transfer from motor.

There are two modes by which we can increase heat transfer inside pod.

1) Increase radiative heat transfer.

2) Increase convective heat transfer.

INCREASING RADIATIVE HEAT TRANSFER:

Heat transfer by radiation between two surfaces at different temperature is given by the StefanBoltzmann

equation:

q = ??A (TH

4

– TC

4

)

Where

? = Stefan-Boltzmann Constant=5.67*E-08 (W/m2K

4

)

TH = temperature of the hot surface(K)

Tc = temperature of cooler surface (K)

? = emissivity

A= surface area of heat sink

Now this equation indicates that the temperature difference between two surface has the

governing effect in the radiative heat transfer. But there are other constrains as emissivity and

surface area of heat sink.

But inside pod, as heat transfer occurs between motor and our surface, surface temperature will

rise constantly, and heat transfer will begin to reduce significantly as, it depends on power 4 of

temperature. So, even small temperature rise will reduce the heat transfer rapidly.

Other way to increase radiative heat transfer rate is to increase surface area. But since that can

increase weight of pod also and with limited space available it cannot be done.

But, as mentioned in this article, effective surface area can be increased without increasing

actual area of heat sink surface.

“An innovative is solution must be developed to increase the effective surface. area without

increasing the size of the parts. Microscopic texturing is such a solution. Microscopic surface

texturing not only increases the surface area; it also increases the emissivity of the surface at

the same time. This is because radiation heat transfer is primarily a surface phenomenon. Thus,

certain texturing. processes that provide sufficient control over surface feature morphology can

increase surface emissivity.” {1}

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Article talks about using surface treatments to increase surface area. Chemical etching is one

of the treatments used frequently and another common treatment is Ion beam texturing.

In theory later, technique can be used to generate any type of texture on surface as we required.

Images of textures

“The larger surface area will enhance radiation as well as convection heat transfer, provided

that the texturing can provide an excellent surface for pool boiling heat transfer, which is often

seen in cooling of high power electronics.” {1}

There is one technique which emphasizes on increasing emissivity of surface called Anodizing.

It is an electrochemical process that thickens and toughens the naturally occurring protective

oxide layer on the surface of metal parts.

“To demonstrate the contribution of anodizing in low airflow velocity applications, two ATS

maxi FLOW heat sinks, one anodized and the other non-anodized, were thermally,

characterized using the exact same method. The heat sink tested, ATS-440-C1-R0, has a

footprint of 45 x 38 mm, its overall height is 24 mm and its fin offset on each side is 21

mm. The heat sink was thermally characterized at natural convection and airflow velocities up

to 3 m/s at increments of 0.5 m/s. Figure 3 compares the thermal resistance of the

two heat sinks at different velocities. As shown, the thermal resistance of the anodized heat

sink at all airflow velocities is lower than that of the non-anodized heat sink. However, the

difference is most significant at natural convection and becomes smaller as the airflow

increases.” {1}

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Conclusion:

Reviewing this article, suggests that use of Anodized textured surfaces can increase radiative

heat transfer for the same temperature difference between surfaces, by increasing emissivity

and effective surface area of heat sink surface. This can be used in future for pod, as we can

build a heatsink with anodizes etched surface around casing which contains liquid coolant. Not

only it will increase radiative heat extraction from motor, but it will also increase convective

heat transfer. As in mentioned in article,

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• Now, there is one other article “Enhancing radiative energy transfer through

thermal extraction” which talk about extracting nearfield radiation from emitter via thermal

extractor to increase radiative heat transfer.

As we know One of the fundamental constraints in thermal radiation is the Stefan-Boltzmann

law, which limits the maximum power of far-field radiation to Po = A?T4

, where ? is the

Boltzmann constant, A and T are the area and the temperature of the emitter(blackbody),

respectively. It is said that near-field radiations could have an energy density that is orders of

magnitude greater than the Stefan-Boltzmann law. Unfortunately, such near-field radiation

transfer is spatially confined and cannot carry radiative heat to the far field.

Article talks about “a new concept of thermal extraction was proposed to enhance far-field

thermal emission, which, conceptually, operates on a principle like oil immersion lenses and

light extraction in light-emitting diodes using solid immersion lens to increase light output.”

{2}

Thus, it will allow a black body to radiate more energy which will be able to reach far field

without breaking laws of thermodynamics. Basically, they put the thermal radiation extractor

so close to the emitter body that the distance between emitter and extractor is less the thermal

wavelength. The near field coupling is capable of transferring heat that has more energy density

than predicted by Stefan-Boltzmann equation. Thermal extractor is made of such material that

has high transitivity which allows near-field radiation to reach far field without being absorbed

or emitted by other surfaces. So, energy reached to far field well exceeds energy calculated

Stefan-Boltzmann equation.

The Article consists of series of experiments which consists using different material for thermal

extractor to see if hypothesis made about nearfield radiation will reach far field as extractor is

made of transparent high index materials.

They have run experiments with Thermal extractor made from natural material as well as

structured material. Experiment setup consist of a standard blackbody sphere with a cavity,

inside surface having high reflective index.

Results show that,

1) Thermal extractor must be in optical contact with emitter body meaning distance

between extractor and emitter surface should be less than thermal wavelengths.

Otherwise near field coupling will take place in between space.

2) Physical contact is not necessary, only optical contact. This will reduce or eliminate

conduction between extractor and emitter.

3) the thermal extraction device needs to provide enough radiation channels over the area

of the emitter to ensure that all internal modes of the emitter can out couple, which can

be achieved by making extractor density sates lighter than emitter.

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4) in the extraction device those optical modes that receive radiation from the emitter need

to be accessible to far-field vacuum. This places a constraint on the geometry of the

extraction device.

Example- A transparent high-index slab with a flat surface does not provide thermal

extraction. Even though more radiations can enter the slab, those outside the escape

cone cannot escape to far-field vacuum due to total internal reflection.

Conclusion:

This solution is still in experimental basis as it may not possible to implement it in every

case. To apply this solution, we may have to make adjustment for thermal extractor

which are optically capable of transferring near- field couple to Fairfield outcoupled.

But also, these extractors must have high thermal insulation as in experiment they were

around room temperature whereas motor inside pod will around much higher

temperature. Thus, it is limited by material constraints of thermal extractors.

• A research paper,” Spectrally enhancing near-field radiative heat transfer

by exciting magnetic polariton in SiC gratings” also suggest that near field couples’ extraction

can be enhanced by presence of magnetic resonance or polariton. Magnetic polaritons (MP)

refer to the strong coupling of external electromagnetic waves with the magnetic resonance

excited inside the nanostructures. But presence of electromagnetic waves may hinder motor

output so cannot be used in my situation. Though it has immense application in real world like

in energy-harvesting, near-field imaging, thermal modulation, and thermal switch and

rectification we cannot use it. {3}

There are numerous articles on extracting near filed Radiation, consisting experiments and their

results are included using different material for thermal extractor like

1) Enhancement of near-field radiative heat transfer using polar dielectric thin films

2) Using high temperature liquid salts (Fluoride and Chloride salts)

3) Nanostructures consisting nanoparticles that emit electromagnetic radiation

May be in future we can use these methods with convenient and with sure results.

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INCREASING CONVECTIVE HEAT TRANSFER:

Now as mentioned earlier that we are using a coolant to vacant produced heat which is done

by convection heat transfer mode. But still there are numerous ways to increase convective

heat transfer which have more concrete data that they will work and have been tested by many

experiments. As A review article “Recent Advances in Heat Transfer Enhancements”

Has said that for

“the mechanisms of heat transfer enhancement can be at least one of the following.

(1) Use of a secondary heat transfer surface.

(2) Disruption of the unenhanced fluid velocity.

(3) Disruption of the laminar sublayer in the turbulent boundary layer.

(4) Introducing secondary flows.

(5) Promoting boundary-layer separation.

(6) Promoting flow attachment/reattachment.

(7) Enhancing effective thermal conductivity of the fluid under static conditions.

(8) Enhancing effective thermal conductivity of the fluid under dynamic conditions.

(9) Delaying the boundary layer development.

(10) Thermal dispersion.

(11) Increasing the order of the fluid molecules.

(12) Redistribution of the flow.

(13) Modification of radiative property of the convective medium.

(14) Increasing the difference between the surface and fluid temperatures.

(15) Increasing fluid flow rate passively.

(16) Increasing the thermal conductivity of the solid phase using special nanotechnology

fabrications.” {4}

As we can see first mechanism uses increased surface area technique which is very common

way to have more heat transfer. Now and no. (2) can be done increasing the surface area in

contact with the fluid to be heated or cooled by using fins, intentionally promoting turbulence

in the wall zone employing surface roughness and tall/short fins, and inducing secondary flows

by creating swirl flow using helical/spiral fin geometry and twisted tapes.

Increasing turbulence can be help in some application to increase mixing of fluids, to increase

heat transfer by convection as turbulent flow will have more collisions between fluid particles,

which increase contact time and transfer of energy from one particle to another.

Arman and Rabas discussed the turbulent flow structure as the flow passes over a twodimensional

transverse rib. They noted that eddies are generated above the flow regions,

particularly at the top of the rib and one in the downstream mixing zone. These zones have the

highest heat transfer rate because of the eddies. Which is basically experiment base don of no

(6) mechanism.

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Ding et al conducted an experiment and as result he got that, that fluids containing 0.5wt.%

of carbon nanotubes (CNT) can increase heat transfer by 250% at Re = 800, which is

significant. report. The increases in heat transfer due to presence of nanofluids are basically

following mechanisms, no (7 & 8).

Heat transfer Enhancers:

1) Creating extended surfaces (Fins)

2) Use of Porous Media (As fluid)

3) Use of Nanofluids

4) Use of complex flexible seals

5) Use of vortex generators (to get turbulent flow)

6) Use of ultra-hight thermal conductive materials

Heat Transfer Enhancer Heat Transfer in presence of enhancer/Heat

Transfer in absence of Enhancer

Ratio(unitless)

Fins Inside Tubes 2

Porous Media 12(approx.)

Nano Fluids 3.5

Complex Flexible seals 3

Vortex Generators 2.5

Ultra-high thermal conductivity composite

materials 6

0 2 4 6 8 10 12 14

Fins Inside Tubes

Porous Media

Nano Fluids

Complex Flexible seals

Vortex Generators

Ultra high thermal conductivity composite…

Ratio of HT in presence of Enhancer to absence of enhancers

Enhancers

Heat Transfer Enhancers

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REFERENCES & CITATION:

{1} Radiation Heat Transfer and Surface Area Treatments

Q pedia Thermal Magazine (Q pedia Thermal Magazine Volume II, Issues 1-12 2008)

{2} Enhancing radiative energy transfer through thermal extraction

Tan, Y., Liu, B., Shen, S., & Yu, Z. (2016). Enhancing radiative energy transfer through

thermal extraction. Nanophotonics, 5(1). doi:10.1515/nanoph-2016-0008)

{3} Spectrally enhancing near-field radiative heat transfer

by exciting magnetic polariton in SiC gratings

Yang, Y., & Wang, L. (2016). Spectrally Enhancing Near-Field Radiative Transfer between

Metallic Gratings by Exciting Magnetic Polaritons in Nanometric Vacuum Gaps. Physical

Review Letters, 117(4). doi:10.1103/physrevlett.117.044301

{4} Recent Advances in Heat Transfer Enhancements: A Review Report

Siddique, M., Khaled, A. A., Abdul hafiz, N. I., & Boukhary, A. Y. (2010). Recent Advances

in Heat Transfer Enhancements: A Review Report. International Journal of Chemical

Engineering, 2010, 1-28. doi:10.1155/2010/106461