Kwok Cheung

Investigation of Low Power Operation in a Loop Heat Pipe

This paper presents test results of an experimental study of low power operation in a loop heat pipe. The main objective was to demonstrate how changes in the vapor void fraction inside the evaporator core would affect the loop behavior, The fluid inventory and the relative tilt between the evaporator and the compensation chamber were varied so as to create different vapor void fractions in the evaporator core. The effect on the loop start-up, operating temperature, and capillary limit was investigated. Test results indicate that the vapor void fraction inside the evaporator core is the single most important factor in determining the loop operation at low powers.

Loop Heat Pipe Testing and Analytical Model Verification at the U.S. Naval Research Laboratory

Heat transfer characteristics of a Loop Heat Pipe (LHP) is difficult to predict due to the complex nature of thermal interaction between the LHP itself and the environment it operates in. The overall thermal conductance varies not only with the power input, sink temperature and ambient temperature as expected, but also with the system initial condition and/or previous history of its operation. Hence, the analytical modeling of LHPs often yielded inaccurate results when compared with the actual data. The U.S. Naval Research Laboratory (NRL) recently completed the development of a transient LHP model. Accordingly, NRL carried out an extensive LHP test program, in part, to provide test data for the model verification. In this paper, description of the test program and results of the model verification will be presented.

Start -Up Behavior of an Ammonia Loop Heat Pipe

Loop Heat Pipes (LHP) are becoming important h eat transport devices for space -based thermal control systems simply because the thermal requirements of future spacecraft and satellites outgro w the capabilities of conventional heat pipes. Like heat pipes, LHPs are capillary -pumped two -phase fluid circu lation systems/devices containing no mechanical moving part to wear out or to introduce unwanted vibration to the host system. They are capable of transporting thousands of watts of waste heat over long distance s for heat rejection at remote locations. LHPs are, m ore importantly, maintenance -free . In spite of the increasing usage in recent years, some spacecraft engineers are still hesitant to utilize LHPs for what they believe to be the system start -up difficulties. In particular , the start -up process is sometimes problematic , especially when the evaporator is attached to a large thermal mass. Nevertheless the problem can be easily overcome by employing an electrical heater and/or a thermoelectric cooler to kick -start the loop as demonstrated in a numb er of LHP systems. The U.S. Naval Research Laboratory (NRL) recently carried out a test program of an ammonia LHP to investigate the feasibility of the LHP technology for the Navys future applications. Focus of the test program was placed on the star t-up process and the procedure to kick -start the LHP if it failed to start on its own. Like everything else, understanding the LHP start -up problem was the prerequisite to solving it. Results of the current test program provided valuable information regardi ng the LHP behavior during the start -up process. Nomenclature W p )

Evaluation of a Magnetically-Driven Bearingless Pump for Spacecraft Thermal Management

High-performance capillary heat transport devices such as Loop Heat Pipes (LHPs) and Capillary Pumped Loops (CPLs) are becoming important heat transport devices for spacebased thermal control systems (TCS) simply because the thermal requirements of future spacecraft and satellites outgrow the capabilities of conventional heat pipes. Like heat pipes, LHPs and CPLs contain no mechanical moving part to wear out or to introduce unwanted vibration to the host system. Each of the LHP and CPL technologies has many advantages in its own right. However, a complete TCS may require many specialized thermal control functions that neither LHP nor CPL alone can provide. Moreover, the need for smaller and lightweight TCS demands that the pumping capability of the heat transport loop be at least one-order-of-magnitude higher than those of LHPs and CPLs. The U.S. Naval Research Laboratory proposed the concept of hybrid two-phase capillary/mechanical pumped loop for next-generation space TCS. In the hybrid loop, a mechanical pump augments the capillary pumping head of a multiple-evaporator LHP or CPL. The capillary pumps provide a nearperfect liquid-vapor separation at the evaporators and, therefore, retain the effectiveness of the two-phase (evaporative) heat transfer of the LHP/CPL. A test program was carried out at NRL to assess the capability and reliability/durability of a magnetically-driven bearingless pump manufactured by Advanced Bionics Incorporated for use in the hybrid loop. Results of the test program are presented in this paper to demonstrate the feasibility of the hybrid loop concept for future TCS design.

Non-Intrusive Fluid Flow Measurement Method for Loop Heat Pipes

The ability to accurately calculate the rate of fluid flow in a Loop Heat Pipe (LHP) is essential in the analytical modeling of the loop performance. The first step in the process of validating a LHP model is, therefore, to verify the predicted flow rate against the measured value in the actual loop operation. Since the LHP operational characteristics are sensitive to the system pressure drop, any mechanical measuring device utilized as part of the fluid loop will certainly affect the thermal performance, most likely, in a bad way. Accordingly, the U.S. Naval Research Laboratory (NRL) developed a non-intrusive method of determining the mass flow rate in a single-phase section of the LHP transport lines simply by measuring the time rate of change of its wall temperature. Hence, the LHP flow rate can be determined at any time during operation, whether under a steady state or transient condition. A proofof-concept test program was carried out to demonstrate the accuracy and responsiveness of the NRL flow measurement method. Different algorithms were employed to deduce the flow rate from the measured temperatures. Preliminary data assessment showed that the method produced excellent results at various flow settings for either gasor liquid-phase fluid flow.

Experimental investigation of reducing startup time on capillary pumped loop with EHD assistance

The capillary pump loop (CPL) is the current state-of-the-art space cooling system. It provides higher cooling capacity than most heat pipes, more installation flexibility, and much greater distance of heat transport due to the small diameter of wickless transport lines. Major disadvantages of the CPL include long and complicated startup procedures and the possibility of depriming at high heat input and load variation. The presented work was an experimental study to characterize the startup process for an EHD-assisted CPL system. Startup is achieved by an almost stable differential pressure and average temperature at the evaporator wall. When the electric field is applied, it interacts with the vapor/liquid distribution inside the core and the wick. It also provides an additional pumping effect of liquid to the evaporator surface. As a result, less time is needed to build up the meniscus. Furthermore, the instability-induced EHD pumping at liquid-vapor interface pushes the liquid-vapor interface near the ...

An experimental feasibility study on EHD-assisted capillary pumped loop (CPL)

The capillary pumped loop (CPL) is a passively pumped two-phase heat transport device that has demonstrated performance capabilities up to an order of magnitude greater than heat pipes, which are the current state-of-art. CPL technology has been developed to a near ready state for use as a thermal control device for advanced spacecraft systems. To further improve CPL performance, on a system level, the pumping head generated within the wick material must be enhanced. Utilizing the effect of a phenomenon known as liquid extraction (or EHD pumping); the Electrohydrodynamic (EHD) technique can effectively improve the liquid pumping capacity in a CPL system. EHD uses an electric field that can collect, guide, and pump liquid to the evaporating surface. This paper presents an experimental investigation of the feasibility of using EHD technology for improving CPL performances. The experimental study included EHD-enhanced pumping across a felt material that simulated a CPL wick. The results show more than 80% in...