Steady State Numerical Modeling of Non-Conventional Loop Heat Pipes (LHPs)

2012 ◽  
Author(s):  
Karthik S. Remella ◽  
Frank M. Gerner ◽  
Ahmed Shuja ◽  
Praveen Medis
Keyword(s):  
Heat Transfer ◽  
Steady State ◽  
Single Phase ◽  
Heat Pipes ◽  
Working Fluid ◽  
Vapor Interface ◽  
Loop Heat Pipes ◽  
Led Array ◽  
Fill Volume ◽  
Liquid Vapor

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.

Steady State Parametric Modeling of Non-Conventional Loop Heat Pipes

2013 ◽  
Author(s):  
Karthik S. Remella ◽  
Frank M. Gerner ◽  
Ahmed Shuja
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Steady State ◽  
Transfer Coefficient ◽  
Thermal Energy ◽  
Heat Pipes ◽  
Parametric Model ◽  
Parametric Modeling ◽  
Working Fluid ◽  
Loop Heat Pipes

Loop Heat Pipes (LHPs) are used in many thermal management applications, especially for micro-electronics cooling, because of their ability to passively transport thermal energy from a source to a sink. This paper describes the development of a parametric model for a non-conventional LHP operating in steady state, employed to cool Light Emitting Diodes (LEDs). This device is comprised of a flat evaporator, and a finned circular loop wherein condensation and sub-cooling of the working fluid takes place. Unlike a conventional LHP, this device has no compensation chamber. In the mesh screen of the evaporator, the vapor flow entrains liquid and hence the quality of the two-phase mixture leaving the evaporator (xevap) is less than unity (unlike in a conventional LHP where saturated vapor leaves the evaporator). Since this lower quality (approximately 0.2) results in a smaller ratio of latent energy to sensible energy being removed by the condenser and sub-cooler respectively; the ratio of the length of the sub-cooler to condenser length is significantly larger. This results in more stable and controlled operation of the device. Mathematical models of the evaporator, the condenser and the sub-cooler sections are developed, and two closure conditions are employed in this model. For consistency and accuracy, some parameters in the model, such as the natural convection heat transfer coefficient (h o) and a few thermal resistances in the evaporator, are estimated empirically from test data on the device. The empirically obtained value of the heat transfer coefficient is in very good agreement with correlations from the literature. The parametric model accurately predicts the LED board temperature and other temperatures for a specific amount of thermal energy dissipated by the LEDs.

Effects of Superhydrophobic and Superhydrophilic Surfaces on Heat Transfer and Oscillating Motion of an Oscillating Heat Pipe

2014 ◽  
Vol 136 (8) ◽  
Author(s):  
Tingting Hao ◽  
Xuehu Ma ◽  
Zhong Lan ◽  
Nan Li ◽  
Yuzhe Zhao
Keyword(s):  
Heat Transfer ◽  
Superhydrophobic Surface ◽  
Thin Liquid Film ◽  
Maximum Amplitude ◽  
Line Length ◽  
Copper Surface ◽  
Working Fluid ◽  
Liquid Slug ◽  
Vapor Interface ◽  
Liquid Vapor

The effects of superhydrophobic surface and superhydrophobic and superhydrophilic hybrid surface on the fluid flow and heat transfer of oscillating heat pipes (OHPs) were investigated in the paper. The inner surfaces of the OHPs were hydrophilic surface (copper), hybrid surface (superhydrophilic evaporation and superhydrophobic condensation section), and uniform superhydrophobic surface, respectively. Deionized water was used as the working fluid. Experimental results showed that superhydrophobic surface influenced the slug motion and thermal performance of OHPs. Visualization results showed that the liquid-vapor interface was concave in the OHP with copper surface. A thin liquid film existed between the vapor plug and the wall of the OHP. On the contrary, the liquid-vapor interface took a convex profile in the OHP with superhydrophobic surface and the liquid-vapor interface contact line length in the hybrid surface OHP was longer than that in the uniform superhydrophobic surface OHP. The liquid slug movements became stronger in the hybrid surface OHPs as opposed to the copper OHP, while the global heat transfer performance of the hybrid surface OHPs increased by 5–20%. Comparing with the copper OHPs, the maximum amplitude and velocity of the liquid slug movements in the hybrid surface OHPs increased by 0–127% and 0–185%, respectively. However, the maximum amplitude and velocity of the liquid slug movements in the uniform superhydrophobic OHPs was reduced by 0–100% and 0–100%, respectively. The partial dryout phenomenon took place in OHPs with uniform superhydrophobic surface. The liquid slug movements became weaker and the thermal resistance was increased by 10–35% in the superhydrophobic surface OHPs.

Derivation and Validation of a Figure of Merit for Loop Heat Pipes With Medium Temperature Working Fluids

2016 ◽  
Vol 138 (5) ◽  
Author(s):  
Wukchul Joung ◽  
Jinho Lee ◽  
Sanghyun Lee ◽  
Joohyun Lee
Keyword(s):  
Steady State ◽  
Thermal Performance ◽  
Figure Of Merit ◽  
Saturation Pressure ◽  
Heat Pipes ◽  
Working Fluid ◽  
Medium Temperature ◽  
Working Fluids ◽  
Loop Heat Pipes ◽  
Compensation Chamber

The working fluids of loop heat pipes (LHPs) play an important role in the operation of the LHPs by influencing the operating temperatures and the heat transfer limits. Therefore, the proper selection of a working fluid is a key practice in LHP fabrication, and there has been a high demand for an appropriate index that enables the quantitative comparison of the steady-state thermal performance of the working fluids. In this work, a figure of merit for LHPs was theoretically derived and experimentally verified. In particular, the pressure losses in the LHP operation were balanced with the saturation pressure difference between the evaporator and the compensation chamber to derive the figure of merit. This derived figure of merit for LHPs successfully predicted the steady-state thermal performance of the tested working fluids within the variable conductance regime. In the constant conductance regime, the differences in the condenser cooling capacity and in the liquid subcooling for different working fluids determined the thermal performance of each working fluid. The limitations and prospects of the proposed figure of merit were discussed in detail.

Experimental Investigation on Performance of Pulsating Heat Pipe

2015 ◽  
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.

Two-Phase Flow With Phase Change in Porous Channels

2015 ◽  
Vol 7 (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.

scholarly journals Experimental Study on Thermal Performance of a Loop Heat Pipe with Different Working Wick Materials

Energies ◽  
2021 ◽  
Vol 14 (9) ◽  
pp. 2453
Author(s):  
Kyaw Zin Htoo ◽  
Phuoc Hien Huynh ◽  
Keishi Kariya ◽  
Akio Miyara
Keyword(s):  
Stainless Steel ◽  
Heat Pipes ◽  
Working Fluid ◽  
Stable Operation ◽  
Loop Heat Pipe ◽  
Heater Surface ◽  
Loop Heat Pipes ◽  
Limited Temperature ◽  
Sintered Stainless Steel ◽  
Continuous Heat

In loop heat pipes (LHPs), wick materials and their structures are important in achieving continuous heat transfer with a favorable distribution of the working fluid. This article introduces the characteristics of loop heat pipes with different wicks: (i) sintered stainless steel and (ii) ceramic. The evaporator has a flat-rectangular assembly under gravity-assisted conditions. Water was used as a working fluid, and the performance of the LHP was analyzed in terms of temperatures at different locations of the LHP and thermal resistance. As to the results, a stable operation can be maintained in the range of 50 to 520 W for the LHP with the stainless-steel wick, matching the desired limited temperature for electronics of 85 °C at the heater surface at 350 W (129.6 kW·m−2). Results using the ceramic wick showed that a heater surface temperature of below 85 °C could be obtained when operating at 54 W (20 kW·m−2).

scholarly journals Heat transfer and entropy generation in a microchannel with longitudinal vortex generators using Nanofluids

2020 ◽  
Author(s):  
Amin Ebrahimi ◽  
Farhad Rikhtegar Nezami ◽  
Amin Sabaghan ◽  
Ehsan Roohi
Keyword(s):  
Heat Transfer ◽  
Entropy Generation ◽  
Single Phase ◽  
Pure Water ◽  
Vortex Generators ◽  
Longitudinal Vortex ◽  
Working Fluid ◽  
Two Phase ◽  
Rectangular Microchannel ◽  
Important Conclusion

Conjugated heat transfer and hydraulic performance for nanofluid flow in a rectangular microchannel heat sink with LVGs (longitudinal vortex generators) are numerically investigated using at different ranges of Reynolds numbers. Three-dimensional simulations are performed on a microchannel heated by a constant heat flux with a hydraulic diameter of 160 μm and six pairs of LVGs using a single-phase model. Coolants are selected to be nanofluids containing low volume-fractions (0.5%–3.0%) of Al2O3 or CuO nanoparticles with different particle sizes dispersed in pure water. The employed model is validated and compared by published experimental, and single-phase and two-phase numerical data for various geometries and nanoparticle sizes. The results demonstrate that heat transfer is enhanced by 2.29–30.63% and 9.44%–53.06% for water-Al2O3 and water-CuO nanofluids, respectively, in expense of increasing the pressure drop with respect to pure-water by 3.49%–16.85% and 6.5%–17.70%, respectively. We have also observed that the overall efficiency is improved by 2.55%–29.05% and 9.78%–50.64% for water-Al2O3 and water-CuO nanofluids, respectively. The results are also analyzed in terms of entropy generation, leading to the important conclusion that using nanofluids as the working fluid could reduce the irreversibility level in the rectangular microchannel heat sinks with LVGs. No exterma (minimums) is found for total entropy generation for the ranges of parameters studied.

scholarly journals Experimental Investigation of Embedded Micropin-Fins for Single-Phase Heat Transfer and Pressure Drop

2018 ◽  
Vol 140 (2) ◽  
Author(s):  
Chirag R. Kharangate ◽  
Ki Wook Jung ◽  
Sangwoo Jung ◽  
Daeyoung Kong ◽  
Joseph Schaadt ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Friction Factor ◽  
Integrated Circuit ◽  
Single Phase ◽  
Absolute Error ◽  
Heat Fluxes ◽  
Operating Conditions ◽  
Working Fluid ◽  
Pin Fin

Three-dimensional (3D) stacked integrated circuit (IC) chips offer significant performance improvement, but offer important challenges for thermal management including, for the case of microfluidic cooling, constraints on channel dimensions, and pressure drop. Here, we investigate heat transfer and pressure drop characteristics of a microfluidic cooling device with staggered pin-fin array arrangement with dimensions as follows: diameter D = 46.5 μm; spacing, S ∼ 100 μm; and height, H ∼ 110 μm. Deionized single-phase water with mass flow rates of m˙ = 15.1–64.1 g/min was used as the working fluid, corresponding to values of Re (based on pin fin diameter) from 23 to 135, where heat fluxes up to 141 W/cm2 are removed. The measurements yield local Nusselt numbers that vary little along the heated channel length and values for both the Nu and the friction factor do not agree well with most data for pin fin geometries in the literature. Two new correlations for the average Nusselt number (∼Re1.04) and Fanning friction factor (∼Re−0.52) are proposed that capture the heat transfer and pressure drop behavior for the geometric and operating conditions tested in this study with mean absolute error (MAE) of 4.9% and 1.7%, respectively. The work shows that a more comprehensive investigation is required on thermofluidic characterization of pin fin arrays with channel heights Hf < 150 μm and fin spacing S = 50–500 μm, respectively, with the Reynolds number, Re < 300.

scholarly journals The effect of operating parameters on the heat transfer in the heat pipe

2021 ◽  
Vol 2119 (1) ◽  
pp. 012088
Author(s):  
A. A. Litvintceva ◽  
N. I. Volkov ◽  
N. I. Vorogushina ◽  
V. A. Moskovskikh ◽  
V. V. Cheverda
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Pipe ◽  
Temperature Difference ◽  
Heat Pipes ◽  
Working Fluid ◽  
Operating Parameters ◽  
Temperature Stabilization ◽  
Ir Camera ◽  
Fluid Properties

Abstract Heat pipes are a good solution for temperature stabilization, for example, of microelectronics, because these kinds of systems are without any moving parts. Experimental research of the effect of operating parameters on the heat transfer in a cylindrical heat pipe has been conducted. The effect of the working fluid properties and the porous layer thickness on the heat flux and temperature difference in the heat pipe has been investigated. The temperature field of the heat pipe has been investigated using the IR-camera and K-type thermocouples. The data obtained by IR-camera and K-type thermocouples have been compared. It is demonstrated the power transferred from the evaporator to the condenser is a linear function of the temperature difference between them.

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