Heat Pipe Heat Exchangers are used when the heat exchange occurs between two fluid streams that are kept separate. In some cases, it is desirable to maintain one of the outlet streams at a constant temperature, even with variations in inlet flow rate and temperatures. In this case, Variable Conductance Heat Pipes (VCHPs) can be used in a VCHP Heat Exchanger.
Figure 1. Variable Conductance Heat Pipe (VCHP).
In a VCHP, a Non-Condensable Gas (NCG) is added to the heat pipe, in addition to the working fluid. Depending on the operating conditions, the NCG can block all, some, or none of the available condenser length. When the VCHP is operating, the NCG is swept toward the condenser end of the heat pipe by the flow of the working fluid vapor. At high powers, all of the NCG is driven into the reservoir, and the condenser is fully open; see Figure 1. As the power is lowered, the vapor temperature drops slightly. Since the system is saturated, the vapor pressure drops at the same time. This lower pressure allows the NCG to increase in volume, blocking a portion of the condenser. At very low powers, the vapor temperature and pressure are further reduced, the NGC volume expands, and most of the condenser is blocked. This change in active condenser length minimizes the drop in evaporator and associated electronics temperatures over large changes in power and evaporator sink conditions.
To control the outlet temperature of one stream, a series of VCHPs are linked together in a heat exchanger with the hottest fluid on the bottom; see the animation in Figure 2, and the VCHP Heat Exchanger photograph in Figure 3. The VCHP Heat Exchanger (Passive Thermal Management for a Fuel Cell Reforming Process) was designed to cool a hydrogen stream to around 85°C with hydrogen flow rates varying by factor of 10, and inlet hydrogen temperature by 250°C.
Figure 2. VCHP Heat Exchanger is used to maintain a nearly constant hydrogen gas temperature, while flow rates can vary by a factor of 10, and inlet hydrogen temperature by 250°C. When the temperature of the incoming hydrogen increases, the NCG in more of the VCHPs is pushed towards the reservoir, increasing the heat transfer to the coolant.
Since the desired output temperature was 85°C, the VCHPs were designed so that they were fully open at the hydrogen inlet temperatures, while the condenser was fully blocked by NCG when the hydrogen temperature was 85°C. As shown in Figure 2, the VCHPs near the inlet are fully open, due to the high temperature of the hydrogen. As the hydrogen passes through the heat exchanger, the VCHPs cool the hydrogen by transferring heat to the counter-current coolant water stream. As the temperature approaches 85°C, the VCHPs shut down. The number of active VCHPs is set by the hydrogen inlet temperature and flow rate: most of the VCHPs are active at high inlet temperatures and/or flow rates, while most of the VCHPs are shut down for low inlet temperature and/or flow rates. The entire system is passive, with no control requirements.
Experimental results for the VCHP Heat Exchanger, as well as a Heat Pipe Heat Exchanger are shown in Figure 4, where the hydrogen inlet temperature was varied from 390 to 110°C. The VCHP passively controlled the hydrogen outlet temperature in a roughly 10°C range, and the hydrogen inlet temperature dropped by roughly 250°C. In contrast, the temperature in a heat pipe heat exchanger dropped by more than 50°C over the same range, with the hydrogen outlet too cold at lower inlet temperatures.