Heat pipe?-a thermodynamics problem

[lirexin]

Can anyone help with whis question?
Just provided with the answerw don't know how to get there


A heat pipe is a simple device which can be used to transfer heat over considerable
distances using only small temperature differences. It consists of a sealed tube filled
with an appropriate fuid. The dryness fraction of the fuid is usually about 0.1. If
the tube is then supported with one end above the other, and the lower end heated,
then that part of working substance which is liquid collects there and boils. If the
upper end of the tube is cooled, then the vapour condenses there, and the conden-
sate is returned by gravity to the lower end. The result is a heat transfer from the
bottom to the top of the tube, with no temperature differences.

If a heat pipe filled with ammonia were to be used to transfer heat from tightly
grouped electronic components to remote heat sinks, what would be their internal
pressure?

i) at -10 C (equipment turned o in a cold climate)
ii) at 20 C
iii) at 50 C (maximum operating temperature).

The answers are
i) 0.291 MPa
ii) 0.857 MPa
iii) 2.033 MPa

 

[Studiot]

Hello and welcome lirexin.

Please note that such heat pipes are not filled, or they could not work as you describe since there would be no space for the vapourisation to take place. They are partially filled with the thermodynamic fluid.
Also many types contain a wick so that the vapour travels up the pipe in the clear space and the fluid is returned by the wick.

In order to calculate the thermodynamic properties we need to solve the equations of state for the system. This requires a knowledge of a minimum of two of the properties linked by these equations and you have only supplied one viz temperature. In fact we also need to know the quantity (mass) of fluid (ammonia) and the internal volume of the pipe.

You can make the following simplifying assumptions.

When the pipe is initially prepared and sealed, sufficient liquid is placed within to maintain some liquid at all working temperatures. Also the air or other gas included is at standard temperature and pressure (STP).

You can then model the working of the system by noting that at all temperatures the refrigerant vapour pressure will be the saturated vapour pressure, which is available in standard engineering tables.

You can add the change of other gas pressure using the gas law.



go well

 

[Topher925]

I don't think this problem can be solved. Like Studiot said, you need to know the mass/volume fraction of the working fluid in the heat pipe at some given state. Otherwise, I don't think its possible to determine an internal pressure.

When the pipe is initially prepared and sealed, sufficient liquid is placed within to maintain some liquid at all working temperatures. Also the air or other gas included is at standard temperature and pressure (STP).

Heat pipes usually only contain a single working fluid, nothing else. Before they are filled they are subjected to a vacuum remove as much air as possible and/or purged with an inert gas.

 

[AlephZero]

I don't think this problem can be solved.

... unless this is just as complicated way of asking you "what pressure liquifies ammonia gas, at three different temperatures".

 

[lirexin]

Hello and welcome lirexin.

Please note that such heat pipes are not filled, or they could not work as you describe since there would be no space for the vapourisation to take place. They are partially filled with the thermodynamic fluid.
Also many types contain a wick so that the vapour travels up the pipe in the clear space and the fluid is returned by the wick.

In order to calculate the thermodynamic properties we need to solve the equations of state for the system. This requires a knowledge of a minimum of two of the properties linked by these equations and you have only supplied one viz temperature. In fact we also need to know the quantity (mass) of fluid (ammonia) and the internal volume of the pipe.

You can make the following simplifying assumptions.

When the pipe is initially prepared and sealed, sufficient liquid is placed within to maintain some liquid at all working temperatures. Also the air or other gas included is at standard temperature and pressure (STP).

You can then model the working of the system by noting that at all temperatures the refrigerant vapour pressure will be the saturated vapour pressure, which is available in standard engineering tables.

You can add the change of other gas pressure using the gas law.



go well

Thx Studiot that helps a lot

 

[h2oski1326]

... unless this is just as complicated way of asking you "what pressure liquifies ammonia gas, at three different temperatures".

Bingo.

If the entire process (from top to bottom) is two-phase, the fluid is at a constant temperature and pressure. They give you the temperature so...

 

[Studiot]

the fluid is at a constant temperature and pressure.

Not really, but its a good start for a working model.

The refrigerant ( and I don't know any heat pipes that use ammonia, most use an organic cocktail) evaporates rapidly at the cpu temp of 75 - 110 C and condenses at the radiator temp of 30 - 40 C.

This is far from constant fluid temperature.

 

[h2oski1326]

The result is a heat transfer from the
bottom to the top of the tube, with no temperature differences.

Not really, but its a good start for a working model.

The refrigerant ( and I don't know any heat pipes that use ammonia, most use an organic cocktail) evaporates rapidly at the cpu temp of 75 - 110 C and condenses at the radiator temp of 30 - 40 C.

This is far from constant fluid temperature.

In a real heat pipe, yes you are correct. There are local hot spots along with changes in capillary pressure etc. etc.

However, the problem the OP asked about said assume constant temperature as quoted above. It appears this is just a thermodynamic exercise to practice finding the saturation pressure given a temperature.

FWIW, ammonia is a fantastic working fluid for many applications. I'm not sure it would work well in a heat pipe but ammonia is used in many industrial refrigeration applications. If it was not incredibly toxic it would get used more.

 

[Studiot]

For ammonia I think it's more the problem of chemical reaction with a copper pipe.
It is very difficult to determine the working fluid in a heat pipe, that is often a trade secret.

A heat pipe at constant temperature couldn't actually transfer any heat. Sometimes we need to challenge misconceptions in an OP.

 

[h2oski1326]

For ammonia I think it's more the problem of chemical reaction with a copper pipe.
It is very difficult to determine the working fluid in a heat pipe, that is often a trade secret.

A heat pipe at constant temperature couldn't actually transfer any heat. Sometimes we need to challenge misconceptions in an OP.

A Carnot cycle, isentropic, isobaric, and adiabatic processes cannot occur in real systems either and yet every thermo book will make 'unrealistic' assumptions like this all the time during a fundamental exercise.

From my perspective this is clearly a fundamental exercise and the emphasis is not the heat pipe design rather finding the fluid properties. So while your skepticism of a device like this existing is warranted it is also not the point of the exercise.

 

[Studiot]

What thermo book would suggest that heat can be transferred without temperature difference?

Surely they all quote the zeroth law in one form or another?

 

[h2oski1326]

What thermo book would suggest that heat can be transferred without temperature difference?

Surely they all quote the zeroth law in one form or another?

An ideal refrigeration cycle has two constant temperature, constant pressure heat transfer processes. Look up latent heat.

 

[Studiot]

Thanks I know what latent heat is.

I specifically said heat can't be transferred without a temperature difference. That is not the same thing as constant temperature.

I'm sure you know, as well as I do, that if latent heat is taken up at one point in a closed system , it cannot be released at another without a temperature difference between the two points.

This is a fundamental tenet of thermodynamics.

 

[Topher925]

A heat pipe at constant temperature couldn't actually transfer any heat. Sometimes we need to challenge misconceptions in an OP.

This is where I got stuck. If the heat pipe is at a constant uniform temperature, then it is fundamentally impossible for the heat pipe to transfer any heat.

If you want to talk about a pressure vessel at different temperatures, now that's a different story and is definitely doable.

 

[Studiot]

What I think H2oskietc means is that we use the following model

The metalwork of the heatpipe has different forced temperatures at each end, say H(ot) and C(old).

The liquid at H is at temperature (T_H+T_C)/2 + $$\delta$$T

The liquid at C is at temperature (T_H+T_C)/2 - $$\delta$$T

This causes

the liquid at H to vapourise, drawing latent heat from the cpu via the metalwork

and

the vapour at C to condense releasing latent heat to the metalwork which is extracted to environment via the fan.