scholarly journals Experimental Investigation of the Operation Modes of a Flat Loop Heat Pipe

2019 ◽  
pp. 1-12
Author(s):  
A. V. Nedaivozov ◽  
V. N. Afanasiev
Keyword(s):  
Heat Pipe ◽  
Flow Regime ◽  
Heat Load ◽  
Operation Mode ◽  
Heating Surface ◽  
Coolant Flow ◽  
Loop Heat Pipe ◽  
Operation Modes ◽  
Vapor Line ◽  
Heat Loads

The paper presents the experimentally investigated operation modes of a flat loop heat pipe (LHP). The LHP is an efficient heat transfer device operating on the principle of evaporation-condensation cycle and successfully applied in space technology, including cooling heat-stressed components of electronic devices and computer equipment.We have experimentally studied how design parameters of the vapor line and its coolant flow influence on the LHP operation mode and also have determined the causes for emerging oscillatory mode of the LHP operation at low heat load. The paper depicts the experimentally measured temperatures in the LHP characteristic points and the photographs of the coolant flow in the vapor line.Based on the experimental data, we have drawn the following conclusions:A vapor-liquid coolant flow in the vapor line in the range of the heat loads under consideration has been detected. There is no superheating vapour observed.The flow regime of the vapor-liquid mixture depends on both the heat load and the vapor pipe diameter. The decrease in the internal diameter of the investigated vapor line section from 7 mm to 4 mm led to the increase of its vapor content and to the decrease of the heating surface temperature when the heat loads were above 80 W. For example, the temperature of the heating surface T1 decreased from 109.5 °С to 100 °С at a heat load of 110 W. Reducing the heat load from 80 W to 60 W leads to a change in the flow regime of the vapor-water mixture from the annular to the slug regime. Found that at low heat loads (up to 40 W), there is no LHP loop operation observed. Periodic fluctuations in the water level in the vapor line are detected. The LHP operates in thermo-syphon mode. For these heat loads, the influence of the vapor line diameter on the thermal state of the LHP is not observed.Found that at low heat loads the LHP operation mode depends only on the flow regime of the coolant in the vapor line. With the annular regime of the coolant flow in the vapor line, a stationary mode of operation of the LHP is observed. When changing the flow regime of the coolant from the annular to the slug, the LHP operation mode is changed from stationary to oscillatory.

Effect of Sintering Temperature and Time in Nickel Wick for Loop Heat Pipe with Flat Evaporator

2014 ◽  
Vol 602-605 ◽  
pp. 528-532
Author(s):  
Shen Chun Wu ◽  
Chang Yu Wu ◽  
Weie Jhih Lin ◽  
Jia Ruei Chen ◽  
Yau Ming Chen
Keyword(s):  
Heat Pipe ◽  
Constant Temperature ◽  
Heat Load ◽  
Sintering Temperature ◽  
Performance Testing ◽  
Effective Porosity ◽  
Temperature Curve ◽  
Loop Heat Pipe ◽  
Maximum Heat ◽  
Load Increase

This paper specifically addresses the effect of changing the constant temperature region of the sintering temperature curve in manufacturing nickel powder capillary structure (wick) on the performance of a flat loop heat pipe (FLHP). The sintering temperature curve is composed of three regions: a region of increasing temperature, a region of constant temperature, and a region of decreasing temperature, with the sintering time and temperature in the region of constant temperature having significant effect on the permeability of the wick. In this study, for wick manufacturing the temperatures in this region tested range from 550°C to 650°C and the time from 30 minutes to 60 minutes. The properties and internal parameters of the wick are measured, and the wick is placed into FLHP for performance testing. Experimental results show that at sintering temperature of 550°C and lasting about 45 minutes, maximum heat load is 200W, minimum thermal resistance is 0.32°C/W, permeability is , porosity is 66%, effective porosity is 3.8and heat flux is around 21W/cm2; related literatures have only reported maximum heat load increase of 25%.

scholarly journals Influence of coolant flow rates on the heat exchanger parameter at variable operation modes

Vestnik MGSU ◽  
2019 ◽  
pp. 621-633 ◽  
Author(s):  
Tatyana A. Rafalskaya ◽  
Valery Ya. Rudyak
Keyword(s):  
Heat Exchanger ◽  
Heat Exchangers ◽  
Operation Mode ◽  
Coolant Flow ◽  
Flow Rates ◽  
Variable Heat ◽  
Operation Modes ◽  
Mathcad Software ◽  
The Given ◽  
Supply Systems

Introduction. Being used in various industries, heat exchangers most often work under conditions of variable coolant flows and temperatures. At the same time, the existing theories of calculating the heat exchanger operation modes are based on the use of constant unitless parameters at any operation mode. Taking into account the effect of coolant rates on the heat transfer coefficient of the heat exchangers, the given relations are bound to specific types of heat exchangers and can only be used at constant coolant temperatures. The purpose of this study is to obtain expressions for determining the effect of coolant flow rates on the variable heat exchanger parameter. Materials and methods. The main variable operation modes for water-to-water heat exchangers used in heat supply systems are determined. Using simulation in the PTC Mathcad software, dependencies describing the change in the heat exchanger parameter for all the considered variable operation modes are defined. This made it possible to obtain a general formula for the change in the heat exchanger parameter for varying coolant flow rates. Coefficients in this formula take into consideration the effect of coolant temperatures, which cannot be known when calculating variable conditions, especially when the interconnected heat exchangers are operating. Results. To test applicability of the existing relations describing the change in the heat exchanger parameter and of obtained formula, a large number of heat exchangers is calculated at variable operation modes. Comparison with the simulation results shows that the correlations of heat exchanger theories work well at the mode with constant coolant temperatures only, while their use at other operation modes can lead to large calculation errors. Conclusions. The obtained formula allows finding the effect of coolant flow rates on the variable heat exchanger parameter. The formula can be used to predict the operation modes of large systems including a large number of various-type heat exchangers.

Investigation of Loop Heat Pipe Startup Using Liquid Core Visualization

2008 ◽  
Author(s):  
B. P. d’Entremont ◽  
J. M. Ochterbeck
Keyword(s):  
Heat Pipe ◽  
Liquid Core ◽  
High Heat ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Two Phase ◽  
Buoyancy Driven Flows ◽  
Compensation Chamber ◽  
Fill Level ◽  
Heat Loads

In this investigation, a Loop Heat Pipe (LHP) evaporator has been studied using a borescope inserted through the compensation chamber into the liquid core. This minimally intrusive technique allows liquid/vapor interactions to be observed throughout the liquid core and compensation chamber. A low conductivity ceramic was used for the wick and ammonia as the working fluid. Results indicate that buoyancy driven flows, both two-phase and single-phase, play essential roles in evacuating excess heat from the core, which explains the several differences in performance between horizontal and vertical orientations of the evaporator. This study also found no discernable effect of the pre-start fill level of the compensation chamber on thermal performance during startup at moderate and high heat loads.

Experimental Investigation of the Miniature Loop Heat Pipe With Flat Evaporator

2005 ◽  
Author(s):  
Randeep Singh ◽  
Aliakbar Akbarzadeh ◽  
Masataka Mochizuki ◽  
Thang Nguyen ◽  
Vijit Wuttijumnong
Keyword(s):  
Thermal Resistance ◽  
Heat Pipe ◽  
Thermal Control ◽  
Capillary Forces ◽  
High Heat ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Wick Structure ◽  
Flat Evaporator ◽  
Heat Loads

Loop heat pipe (LHP) is a very versatile heat transfer device that uses capillary forces developed in the wick structure and latent heat of evaporation of the working fluid to carry high heat loads over considerable distances. Robust behaviour and temperature control capabilities of this device has enable it to score an edge over the traditional heat pipes. In the past, LHPs has been invariably assessed for electronic cooling at large scale. As the size of the thermal footprint and available space is going down drastically, miniature size of the LHP has to be developed. In this paper, results of the investigation on the miniature LHP (mLHP) for thermal control of electronic devices with heat dissipation capacity of up to 70 W have been discussed. Copper mLHP with disk-shaped flat evaporator 30 mm in diameter and 10 mm thickness was developed. Flat evaporators are easy to attach to the heat source without any need of cylinder-plane-reducer saddle that creates additional thermal resistance in the case of cylindrical evaporators. Wick structure made from sintered nickel powder with pore size of 3–5 μm was able to provide adequate capillary forces for the continuos circulation of the working fluid, and successfully transport heat load at the required distance of 60 mm. Heat was transferred using 3 mm ID copper tube with vapour and liquid lines of 60 mm and 200 mm length respectively. mLHP showed very reliable start up at different heat loads and was able to achieve steady state without any symptoms of wick dry-out. Tests were conducted on the mLHP with evaporator and condenser at the same level. Total thermal resistance, R total of the mLHP came out to be in the range of 1–4°C/W. It is concluded from the outcomes of the investigation that mLHP with flat evaporator can be effectively used for the thermal control of the electronic equipments with restricted space and high heat flux chipsets.

Experimental investigation of a loop heat pipe with R245fa and R1234ze (E) as working fluids

2021 ◽  
pp. 1-17
Author(s):  
Guangming Xu ◽  
Rongjian Xie ◽  
Nanxi Li ◽  
Cheng Liu
Keyword(s):  
Thermal Resistance ◽  
Heat Pipe ◽  
Characteristic Temperature ◽  
Thermal Characteristics ◽  
Heat Pipes ◽  
Loop Heat Pipe ◽  
Working Fluids ◽  
Loop Heat Pipes ◽  
Heat Transfer Capacity ◽  
Heat Loads

Abstract Two kinds of new refrigerant-R1234ze (E) and R245fa were discussed as substitutes or supplements to traditional working fluids of loop heat pipes based on their favorable thermophysical properties and characteristics such as being safe and environmentally friendly. Thermal characteristics of a loop heat pipe with sintering copper wick at different charging ratios were experimentally investigated under variable heat loads. The results showed that the optimal charging ratio in the loop heat pipe range from 65% to 70%, and at this charging level, the R1234ze(E) system had better start-up response, while the R245fa system presented a stronger heat transfer capacity. The characteristic temperature of R1234ze(E) system was below 35 °C, and the corresponding thermal resistance was 0.08 K/W ~ 1.62 K/W under heat loads ranging from 5 W to 40 W. The thermal resistance of the R245fa system was 0.18 K/W ~ 0.91 K/W under heat loads of 10 W ~ 60 W, and the operating temperature was below 60 °C. The loop heat pipes charged with the proposed new refrigerants exhibit superb performance in room temperature applications, making them beneficial for enhancing the performance of electronics, and could provide a distinctive choice for the cooling of small-sized electronics especially.

Analysis of Vapor Blanket Formation in the Porous Wick of a Loop Heat Pipe With High Heat Load

2013 ◽  
Author(s):  
X. M. Huang ◽  
X. Jin ◽  
B. B. Chen ◽  
W. Liu
Keyword(s):  
Mass Transfer ◽  
Heat Flux ◽  
Heat Pipe ◽  
Heat And Mass Transfer ◽  
Heat Load ◽  
High Heat ◽  
Loop Heat Pipe ◽  
Porous Wick ◽  
High Heat Load ◽  
Theoretical Results

A loop heat pipe has different transport mechanisms depending on heat flux. The interface of liquid and vapor cannot maintain at the surface of the wick when heat flux is high, and a vapor blanket will form in the wick. To investigate when the vapor blanket appears and how it affects heat and mass transfer in the system is very import to minimize the device. A mathematical model of heat and mass transfer in the evaporator, coupled with analysis of fluid flow in the loop, is developed in the paper. The model is applied to calculate the critical heat load that the vapor blanket forms, and to analyze how the blanket delays. A comparison of theoretical results and experimental measurements is further presented. The consistence of the results validates the model and the mechanisms.

Improvement of Heat Transfer Performance of Loop Heat Pipe for Electronic Devices

2011 ◽  
Author(s):  
Tomonao Takamatsu ◽  
Katsumi Hisano ◽  
Hideo Iwasaki
Keyword(s):  
Heat Pipe ◽  
Heat Transport ◽  
Thermal Energy ◽  
Heat Load ◽  
Cooling System ◽  
Electronic Devices ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Reserve Area ◽  
Heat Transport Capability

In this paper is presented the results on performance of the cooling model using Loop Heat Pipe (LHP) system. In recent years, ever-ending demand of high performance CPU led to a rapid increase in the amount of heat dissipation. Consequently, thermal designing of electronic devices need to consider some suitable approach to achieve high cooling performance in limited space. Heat Pipe concept is expected to serve as an effective cooling system for laptop PC, however, it suffered from some problems as follows. The heat transport capability of conventional Heat Pipe decreases with the reduction in its diameter or increase in its length. Therefore, in order to use it as cooling system for future electronic devices, the above-mentioned limitations need to be removed. Because of the operating principle, the LHP system is capable of transferring larger amount of heat than conventional heat pipes. However, most of the LHP systems suffered from some problems like the necessity of installing check valves and reservoirs to avoid occurrence of counter flow. Therefore, we developed a simple LHP system to install it on electronic devices. Under the present experimental condition (the working fluid was water), by keeping the inside diameter of liquid and vapor line equal to 2mm, and the distance between evaporator and condenser equal to 200mm, it was possible to transport more than 85W of thermal energy. The thickness of evaporator was about 5mm although it included a structure to serve the purpose of controlling vapor flow direction inside it. Successful operation of this system at inclined position and its restart capability are confirmed experimentally. In order to make the internal water location visible, the present LHP system is reconstructed using transparent material. In addition, to estimate the limit of heat transport capability of the present LHP system using this thin evaporator, the air cooling system is replaced by liquid cooling one for condensing device. Then this transparent LHP system could transport more than 100W of thermal energy. However, the growth of bubbles in the reserve area with the increase in heat load observed experimentally led to an understanding that in order to achieve stable operation of the LHP system under high heat load condition, it is very much essential to keep enough water in the reserve area and avoid blocking the inlet with bubbles formation.

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