Liquid-Vapor Transitions in Mercury and Sodium Gas-Controlled Heat-Pipes

Loop heat pipes (LHPs) transport energy from an evaporator to a condenser in the form of latent heat. In conventional LHPs, the vapor pressure is significantly higher than the liquid pressure across the liquid-vapor interface due to the small pores and the corresponding capillary forces in the wick. This large pressure difference transports the single phase vapor after evaporation from the evaporator to the condenser and once the vapor is condensed, a single phase liquid from the condenser back to the evaporator. This current work involves the development of a steady state design model of the LHP system consisting of a planar evaporator package and a finned copper tube loop, which serves as an air-cooled condenser. Although evaporation due to the heat transfer creates the pressure in the vapor which drives the flow, contrasting to the conventional loop heat pipes, the pressure drop across the liquid-vapor interface is much smaller. A positive hydrostatic head is applied to the liquid above the wick and there is entrainment of liquid from the wick in the evaporator. Therefore, the fluid flow leaving the evaporator package is a two-phase flow, assumed to be saturated liquid and saturated vapor in equilibrium. The primary objective of this non-conventional LHP device is to remove the thermal energy dissipated from a Light Emitting Diode (LED) array. A major portion of this energy causes evaporation of the working fluid within the wick. The remaining energy reheats the liquid in both the liquid return line and within the evaporator package. The evaporator package is modeled as a one-dimensional thermal resistance network and these resistances are empirically determined from experiments. It is found that the convective heat transfer co-efficient of air plays a pivotal role in the heat dissipation and hence, is empirically determined in this work. This value is fairly agreeable with the Nusselt number correlation on the air side developed by Hahne et al. [1]. A mass balance relates the fill volume with the length of the condenser. The temperatures within the LHP are predicted by applying the principle of conservation of energy over the evaporator, the condenser and the sub-cooler sections of the heat exchanger loop. Finally, this LHP model predicts an approximate fill volume necessary for the LHP to operate properly.

Download Full-text

Combined Liquid Vapor Flow in Cylindrical Heat Pipes with Modified Internal Geometry

Advances in Heat Pipe Technology ◽

10.1016/b978-0-08-027284-9.50036-x ◽

1982 ◽

pp. 349-358

Author(s):

K.A.R. Ismail ◽

N. Murcia

Keyword(s):

Heat Pipes ◽

Vapor Flow ◽

Internal Geometry ◽

Liquid Vapor

Download Full-text

Analysis of cylindrical heat pipes incorporating the effects of liquid–vapor coupling and non-Darcian transport—a closed form solution

International Journal of Heat and Mass Transfer ◽

10.1016/s0017-9310(99)00017-4 ◽

1999 ◽

Vol 42 (18) ◽

pp. 3405-3418 ◽

Cited By ~ 59

Author(s):

N. Zhu ◽

K. Vafai

Keyword(s):

Closed Form ◽

Closed Form Solution ◽

Heat Pipes ◽

Form Solution ◽

Liquid Vapor

Download Full-text

Experimental Investigation on Performance of Pulsating Heat Pipe

ASME 2015 Gas Turbine India Conference ◽

10.1115/gtindia2015-1362 ◽

2015 ◽

Cited By ~ 3

Author(s):

Tarigonda Hari Prasad ◽

Pol Reddy Kukutla ◽

P. Mallikarjuna Rao ◽

R. Meenakshi Reddy

Keyword(s):

Heat Transfer ◽

Heat Transfer Coefficient ◽

Thermal Resistance ◽

Transfer Coefficient ◽

Heat Pipe ◽

Heat Pipes ◽

Working Fluid ◽

Pulsating Heat Pipe ◽

The Given ◽

Liquid Vapor

Pulsating heat pipes (PHP) receives heat from the working fluid distributes itself naturally in the form of liquid–vapor system, i.e., receiving heat from one end and transferring it to other end by a pulsating action of the liquid–vapor system. Pulsating heat pipes have more advantages than other heat pipes. The problem identified is, to calculate the performance of the pulsating heat pipes with respect to different inclinations using various parameters. In this paper, experiment on performance of closed single loop pulsating heat pipe (CLPHP) using water as a working fluid is considered. The parameters such as thermal resistance (Rth), heat transfer coefficient (h), and variation of temperature with respect to time for the given input at different inclinations such as 0°, 45°, and 90° are taken for the present work. Water is used as the working fluid and is subjected to 50% filling ratio and vacuumed at a pressure of 2300Pa. The performance is calculated at different inclinations of the CLPHP with single turn/loop. The factors such as heat transfer coefficient, thermal resistance, time taken for heating the pulsating heat pipe with the given input are calculated. Finally, it has been concluded that is preferable orientation for PHP and it was found be at vertical orientation i.e., at 90° inclination, because more pulsating action is taken place at this inclination and henceforth, heat transfer rate is faster at this inclination.

Download Full-text

Two-Phase Flow With Phase Change in Porous Channels

Journal of Thermal Science and Engineering Applications ◽

10.1115/1.4029458 ◽

2015 ◽

Vol 7 (2) ◽

Cited By ~ 2

Author(s):

Gaurav Tomar

Keyword(s):

Heat Transfer ◽

Nusselt Number ◽

Phase Change ◽

Heat Pipes ◽

Porous Channel ◽

Two Phase ◽

Vapor Interface ◽

Interface Location ◽

Heat Loads ◽

Liquid Vapor

Phase change heat transfer in porous media finds applications in various geological flows and modern heat pipes. We present a study to show the effect of phase change on heat transfer in a porous channel. We show that the ratio of Jakob numbers based on wall superheat and inlet fluid subcooling governs the liquid–vapor interface location in the porous channel and below a critical value of the ratio, the liquid penetrates all the way to the extent of the channel in the flow direction. In such cases, the Nusselt number is higher due to the proximity of the liquid–vapor interface to the heat loads. For higher heat loads or lower subcooling of the liquid, the liquid–vapor interface is pushed toward the inlet, and heat transfer occurs through a wider vapor region thus resulting in a lower Nusselt number. This study is relevant in the designing of efficient two-phase heat exchangers such as capillary suction based heat pipes where a prior estimation of the interface location for the maximum heat load is required to ensure that the liquid–vapor interface is always inside the porous block for its operation.

Download Full-text