Heat pipe heat transfer: Wonderful heat transfer principle

This post is all about the wonderful heat transfer process sets inside the heat pipe as the pipe is heated hidden from our eyes and the limitations. Spacecraft, Computer systems, Ventilation heat recovery, Nuclear power conversion, and heat pipe heat exchangers

What is a heat pipe?

At a fundamental level, a heat pipe combines the principles of both thermal conductivity and phase transition to effectively transfer heat between two solid interfaces. The two solid interfaces in a heat pipe are the metal and a lined porous material called a wick. Thermal conductivity along the direction of heat transport is generally at least four to five orders of magnitude greater than the thermal conductivity of copper.

Application of heat pipes: ?Five typical examples

A heat pipe heat exchanger contains several heat pipes of which each acts as an individual heat exchanger itself. This increases efficiency, life span, and safety. In case one heat pipe breaks, only a small amount of liquid is released. With one broken heat pipe, the heat pipe heat exchanger still remains operable.

How does heat transfer through a pipe?

A heat pipe is not an ordinary pipe as many of us might think. A heat pipe is a specially constructed heat-transfer device consisting of a sealed vessel usually made from aluminium or copper. It is a closed structure whose internal surface is lined with a thin layer of porous material, usually referred to as a wick and working fluid charged under a vacuum condition

The container may have a cylindrical shape or any other shape that can be conveniently manufactured. The pores of the wick [lined porous material] are filled with a working liquid appropriate to the application, and the vapour of the liquid occupies the remaining internal volume. Energy absorbed and released from the phase change of the fluid in the heat pipe allows an extremely fast heat transfer.

Therefore, since the liquid and its vapour coexist in equilibrium, the pressure inside the container is equal to the vapour pressure corresponding to the saturation conditions.

?Wonderful heat transfer process sets in the heat pipe as the pipe is heated.

This relatively simple configuration allows for a very efficient heat transfer from one end of the heat pipe to the other, following a quite simple heat transfer mechanism. As heat is applied to one end [LHS of the image], the evaporator, the working liquid evaporates from the wick, while the removal of heat from some other portion of the surface (the condenser) causes the vapour to condensate on the wick. The pressure gradient resulting from the accumulation of vapour at one end of the heat pipe and its depletion at the other end causes the vapour to flow through the core region of the container (the vapour space). But, as the liquid evaporates, it retreats into the wick pores, then the meniscus there is depressed and the liquid pressure drops below the pressure of the adjacent vapour. At the other end, condensation takes place, so that the working liquid fills in the wick, tending to maintain a flat surface without any depression of the pressure in the liquid. Due to capillary forces, the result is a pressure gradient in the liquid that causes the working liquid to flow through the wick toward the evaporator end, in the opposite direction to that of the flowing vapour in the core region, completing the flow circuit.

The pressure variations in the vapour core are normally small and, therefore, the heat pipe temperature is almost uniform and close to the saturated vapour temperature corresponding to the vapour pressure (heat transfer through a heat pipe is virtually isothermal because the vapour pressure drop is usually of the order of 1% or less). Therefore, the heat pipe can be considered an extra-high thermal conductivity device, with reference to Fourier’s law, as the effective thermal conductivity along the direction of heat transport is generally at least four to five orders of magnitude greater than the thermal conductivity of copper.

Working fluid of heat pipe

Working fluids are chosen according to the temperatures at which the heat pipe must operate, with examples ranging from liquid helium for extremely low temperature applications (2–4 K) to mercury (523–923 K), sodium (873–1473 K) and even indium (2000–3000 K) for extremely high temperatures. The vast majority of heat pipes for room temperature applications use ammonia (213–373 K), alcohol (methanol (283–403 K) or ethanol (273–403 K), or water (298–573 K) as the working fluid. Copper/water heat pipes have a copper envelope, use water as the working fluid and typically operate in the temperature range of 20 to 150 °C Water heat pipes are sometimes filled by partially filling with water, heating until the water boils and displaces the air, and then sealed while hot.

Wick structure

Wick structures used in heat pipes include sintered metal powder, screen, and grooved wicks, which have a series of grooves parallel to the pipe axis.

Heat pipe materials and working fluids

Heat pipes are designed for very long-term operation with no maintenance, so the heat pipe wall and wick must be compatible with the working fluid. Some material/working fluids pairs that appear to be compatible are not. For example, water in an aluminium envelope will develop large amounts of non-condensable gas over a few hours or days, preventing normal operation of the heat pipe.

The most commonly used envelope (and wick)/fluid pairs include:

Copper envelop with water working fluid for electronics cooling. This is by far the most common type of heat pipe.

Copper or steel envelope with refrigerant R134a working fluid for energy recovery in HVAC systems.

Aluminium envelops with ammonia working fluid for spacecraft thermal control.

Superalloy envelops with alkali metal (caesium, potassium, sodium) working fluid for high-temperature heat pipes, most commonly used for calibrating primary temperature measurement devices.

Heat pipes contain no mechanical moving parts and typically require no maintenance, though non-condensable gases that diffuse through the pipe's walls, resulting from the breakdown of the working fluid or as impurities extant in the material, may eventually reduce the pipe's effectiveness at transferring heat.

The advantage of heat pipes over many other heat-dissipation mechanisms is their great efficiency in transferring heat. A pipe one inch in diameter and two feet long can transfer 3.7 kW (12.500 BTU per hour) at 1,800 °F (980 °C) with only 18 °F (10 °C) drop from end to end.

?Important Limitations

Capillary Limit

It occurs when the pumping rate is not sufficient to provide enough liquid to the evaporator section. This is due to the fact that the sum of the liquid and vapor pressure drops exceeds the maximum capillary pressure that the wick can sustain. The maximum capillary pressure for a given wick structure depends on the physical properties of the wick and working fluid. Any attempt to increase the heat transfer above the capillary limit will cause dry out in the evaporator section, where a sudden increase in wall temperature along the evaporator section takes place.

Sonic Limit

The evaporator and condenser sections of a heat pipe represent a vapor flow channel with mass addition and extraction due to evaporation and condensation, respectively. The vapor velocity increases along the evaporator and reaches a maximum at the end of the evaporator section. The limitation of such a flow system is similar to that of a converging-diverging nozzle with a constant mass flow rate, where the evaporator exit corresponds to the throat of the nozzle. Therefore, one expects that the vapor velocity at that point cannot exceed the local speed of sound. This choked flow condition is called the sonic limitation. The sonic limit usually occurs either during heat pipe startup or during steady-state operation when the heat transfer coefficient at the condenser is high. The sonic limit is usually associated with liquid-metal heat pipes due to high vapor velocities and low densities. Unlike the capillary limit, when the sonic limit is exceeded, it does not represent a serious failure. The sonic limitation corresponds to a given evaporator end cap temperature. Increasing the evaporator end cap temperature will increase this limit to a new higher sonic limit. The rate of heat transfer will not increase by decreasing the condenser temperature under the choked condition. Therefore, when the sonic limit is reached, further increases in the heat transfer rate can be realized only when the evaporator temperature increases. Operation of heat pipes with a heat rate close to or at the sonic limit results in a significant axial temperature drop along the heat pipe.

?Credit: Google