Amrit Ambirajan

Loop Heat Pipes: A Review of Fundamentals, Operation, and Design

The loop heat pipe (LHP) is a passive two-phase heat transport device that is gaining importance as a part of spacecraft thermal control systems and also in applications such as in avionics cooling and submarines. A major advantage of a loop heat pipe is that the porous wick structure is confuned to the evaporator section, and connection between the evaporator and condenser sections is by smooth tubes, thus minimizing pressure drop. A brief overview of loop heat pipes with respect to basic fundamentals, construction details, operating principles, and typical operating characteristics is presented in this paper. Finally, the paper presents the current developments in modeling of thermohydraulics and design methodologies of LHPs.

Thermohydraulic Modeling of Capillary Pumped Loop and Loop Heat Pipe

Capillary pumped loop (CPL) and loop heat pipe (LHP) are passive heat transport devices that are gaining importance as a part of the thermal control system of modern high power spacecraft. A mathematical model to simulate the thermohydraulic performance of CPLs and LHPs is required for the design of such a thermal control system. In this study a unified mathematical model to estimate thermal and hydraulic performance of a CPL and an LHP with a two-phase or a hard-filled reservoir is presented. The steady-state model is based on conservation of energy and mass in the system. Heat exchange between the loop and the surroundings and pressure drops in the loop are calculated. This study presents the results of numerical studies on a CPL and an LHP. The constant conductance regime in a CPL or an LHP occurs when the reservoir is hard-filled. It also occurs in an LHP if the condenser is fully used. The heat leak across the wick becomes significant in a hard-filled LHP because the core is no longer saturated and hence the mass flow rate must be calculated using an energy balance on the outer surface of the wick.

Theoretical Studies of Hard Filling in Loop Heat Pipes

Loop heat pipe is a passive two-phase heat transport device that is gaining importance as a part of spacecraft thermal control systems and also in applications (such as in avionic cooling and submarines). Hard fill of a loop heat pipe occurs when the compensation chamber is full of liquid. A theoretical study is undertaken to investigate the issues underlying the loop beat pipe hard-fill phenomenon. The results of the study suggest that the mass of charge and the presence of a bayonet have significant impact on the loop heat pipe operation. With a largern mass of charge, a loop heat pipe hard fills at a lower heat load. As the heat load increases, there is a steep rise in the loop heat pipe operating temperature. In a loop heat pipe with a saturated compensation chamber, and also in a hard-filled loop heat pipe without a bayonet, the temperature of the compensation chamber and that of the liquid core are nearly equal. When a loop heat pipe with a bayonet hard fills, the compensation chamber and the evaporator core temperatures are different.

Theoretical and Experimental Studies on an Ammonia-Based Loop Heat Pipe With a Flat Evaporator

An ammonia loop heat pipe (LHP) with a flat plate evaporator is developed and tested. The device uses a nickel wick encased in an aluminum-stainless steel casing. The loop is tested for various heat loads and different sink temperatures, and it demonstrated reliable startup characteristics. Results with the analysis of the experimental observation indicate that the conductance between the compensation chamber and the heater plate can significantly influence the operating temperatures of the LHP. A mathematical model is also presented which is validated against the experimental observations.

Experimental Investigation of the Multiple Scattering of a Polarized Laser Beam

In this paper the forward-scattered diffuse radiation produced by a circularly polarized laser beam impinging on a e nite plane-parallel scattering medium is studied experimentally. The radial variation of the forward-scattered Stokes vector is analyzed. Values of the degree of circular and linear polarization of the scattered radiation computed using experimental data are compared with values obtained using a Monte Carlo code. Trends with respect to the size parameters of the scatterers and the optical thickness are also presented. It was observed that the diffuse radiation is strongly polarized, even at large optical radii. This is true for all the size parameters and optical depths of the medium studied.

Effects of mass of charge on loop heat pipe operational characteristics

Experimental results on a loop heat pipe, using R134a as the working fluid, indicates that the liquid inventory in the compensation chamber can significantly influence the operating characteristics. The large liquid inventory in the compensation chamber, under terrestrial conditions, can result in loss of thermal coupling between the compensation chamber and the evaporator core. This causes the operating temperature to increase monotonically. This phenomenon, which has been experimentally observed, is reported in this paper. A theoretical model to predict the steady-state performance of a loop heat pipe with a weak thermal link between the compensation chamber and the core, as observed in the experiment, is also presented. The predicted and the experimentally determined temperatures correlate well.

Evaporation Heat Transfer Coefficient in a Capillary Pumped Loop and Loop Heat Pipe for Different Working Fluids

Capillary pumped loop (CPL) and loop heat pipe (LHP) are passive two-phase heat transport devices. They have been gaining importance as a part of the thermal control system of spacecraft. The evaporation heat transfer coefficient at the tooth–wick interface of an LHP or CPL has a significant impact on the evaporator temperature. It is also the main parameter in sizing of a CPL or LHP. Experimentally determined evaporation heat transfer coefficients from a three-port CPL with tubular axially grooved (TAG) evaporator and a TAG LHP with acetone, R-134A, and ammonia as working fluids are presented in this paper. The influences of working fluid, hydrodynamic blocks in the core, evaporator configuration (LHP or CPL), and adverse elevation (evaporator above condenser) on the heat transfer coefficient are presented.

Effect of Heat Pipe Figure of Merit on an Evaporating Thin Film

The performance of a two-phase heat transport device such as the loop heat pipe is influenced by the evaporative heat transfer coefficient in the evaporator. From previous experiments with loop heat pipes, it has been observed that fluids with a high heat pipe figure of merit have a high heat transfer coefficient. Considering an evaporating extended thin film, this paper theoretically corroborates this experimental observation by deriving a direct link between the evaporative heat flux at the interface and the fluid figures of merit (namely interline heat flow parameter and heat pipe figure of merit) in the thin film. Numerical experiments with different working fluids clearly show that a fluid with high figure of merit also has a high cumulative heat transfer in the microregion encompassing the evaporating thin film. Thus, a loop heat pipe or heat pipe that uses a working fluid with a high interline heat flow parameter and heat pipe figure of merit will lead to a high evaporative heat transfer coefficient.

Numerical and Experimental Studies of Parasitic Heat Losses in Coldfinger of a Pulse Tube Cryocooler in Off-State Condition

ABSTRACT A pulse tube cryocooler (PTC) for future metrological satellites has been developed at one of the lead centers of the Indian Space Research Organisation in Bangalore, India for cooling on-board Infrared (IR) detectors to 80 K.A study has been conducted on the coldfinger of PTC to understand the off-state heat loads on the cooler by varying the value of gravity numerically in ANSYS FLUENT and experimentally by orienting the setup with respect to gravity. The off-state parasitic losses represent a major heat load in on-board applications that include redundant, viz. nonoperating coolers. To find out the amount of off-state parasitic heat losses in a nonoperating coldfinger of the PTC experimentally, transient warm-up technique was used. Various heat loads were applied experimentally on the cryo-tip at temperatures ranging from 80 to 100 K for determining the parasitic losses. The effect of orientation of PTC on the off-state parasitic heat load with respect to gravity is studied and presented in this paper. Enhancement due to free convection heat flow normalized by gas molecular conduction in pulse tube is analyzed using computational fluid dynamics to verify and compare with experimental results. The best orientation angle where the parasitic is low is when the cold end of the coldfinger of pulse tube cryocooler faces down (0°) and high when the cold end of the coldfinger is oriented to 135°.

The Effect of Directional Radiative Surface Properties on the Performance of Mutually Irradiating Conducting Fins

The problem of mutually irradiating conducting fins is considered in this paper. The problem is analyzed under two types of surface models, namely, diffuse-specular (D-S) model and a directional (Dir) radiative surface property model, for various surface roughnesses and RMS slopes. Metal (aluminum) coated and dielectric (Aluminum oxide) coated surfaces are considered as the two extreme cases. Analysis indicates that dielectric coated surfaces are sensitive to the radiative surface property model while metal coated surfaces are insensitive to the surface property model. Irrespective of the surface property model used metal coated surface show a drastic drop in heat flux for even a slight increase in surface roughness.Copyright