Heat pipe Start Up

From Thermal-FluidsPedia

Figure 1: Transient axial temperature profiles during heat pipe startup: (a) Uniform startup; (b) Frozen startup; (c) Gas-loaded heat pipe startup.

The procedures typically used in designing heat pipes are based on the fact that the heat pipe is in its normal operating state, i.e., nominal temperature at the steady state. However, the heat pipe must be started from the ambient temperature, which is either lower or higher than the operating temperature ^[1][2]. If the startup is too fast, the possibility of overheating the evaporator section can result in damage to the heat pipe. Ideally, the heat pipe should first be started by increasing (or decreasing) the temperature of the entire pipe to its operating temperature. The heat input to the pipe is then slowly increased from zero to the operating input, maintaining a uniform temperature across the length of the pipe. This procedure is not usually feasible in practice. Normally, the heat input increases suddenly from zero to the operating input, without the benefit of increasing the temperature of the entire pipe. This may result in problems occurring in the startup period, which will be discussed here for three different common situations.

Uniform Startup

Figure 1(a) shows the startup of a heat pipe in which the axial temperature is nearly uniform throughout the startup period. This type of startup profile is normally seen in heat pipes in which the working fluid in the wick is in the liquid state, and the vapor is in the continuum state. An example is a copper-water heat pipe starting from ambient temperature to some higher operating temperature. As can be seen, no problems are encountered during the uniform startup.

Frozen Startup

Figure 1(b) presents the case of a heat pipe in which the working fluid within the wick structure is initially frozen, and the vapor space is essentially evacuated. This type of frontal startup is usually found in liquid-metal heat pipes, since liquid metals are in the solid state at room temperature. Frozen startup proceeds as follows: Heat is first conducted through the pipe wall and into the wick structure, increasing the temperature in the evaporator section only. After the working fluid in the evaporator wick is liquefied, evaporation begins to fill the vapor space with vapor. The vapor travels to the adiabatic section and condenses, releasing its latent heat and increasing the adiabatic section temperature. This front continues down the length of the pipe until it reaches the condenser end cap. At that time, the axial temperature distribution starts to become uniform, and the startup process is completed. The presence of large axial temperature gradients indicates that the sonic limit is occurring during frozen startup. In actuality, this is not a limit per se, since the heat pipe will not be damaged during this process. Another more important limit is the frozen startup limit, as previously discussed. The frozen startup limit occurs when more working fluid is evaporated than can be resupplied by the wick structure, due to the fact that the working fluid in the adiabatic and condenser sections is completely frozen. When this occurs, the evaporator section is depleted of liquid and overheats due to dryout.

Gas-Loaded Heat Pipe Startup

The noncondensible gas present in gas-loaded heat pipes gives the startup a frontal character. Figure 1(c) shows the case in which the working fluid in the wick is initially liquid in a gas-loaded heat pipe. Before the heat input is applied in the evaporator, the noncondensible gas is evenly distributed throughout the vapor space. When evaporation occurs, the working fluid vapor drives the gas to the condenser end of the heat pipe. Since the gas and the vapor are essentially separated, the effect of the gas is to block the condenser section from transferring heat to the heat sink. The presence of the gas can be seen in the axial temperature profile, which has an abrupt drop at the vapor-gas interface. More heat is then fed into the pipe, increasing the wall and vapor temperatures within the evaporator and adiabatic sections. This increases the vapor pressure, which compresses the gas in the condenser section. The vapor-gas front moves downstream into the condenser section, eventually unblocking part of the condenser, so that normal operation can take place.

References

1. ↑ Faghri, A., 2012, "Review and Advances in Heat Pipe Science and Technology," Journal of Heat Transfer, 134(12), 123001. http://dx.doi.org/10.1115/1.4007407
2. ↑ Faghri, A., 1995, Heat Pipe Science and Technology, 1st ed., Taylor & Francis, Washington, D.C.