scholarly journals Numerical Investigation on the Dielectric Working Fluid Effect on the Flow and Thermal Parameters of an Electrohydrodynamic Flat Heat Pipe

2020 ◽  
Vol 79 (1) ◽  
pp. 128-152
Author(s):  
Imène Saad ◽  
Samah Maalej ◽  
Mohamed Chaker Zaghdoudi
Keyword(s):  
Pressure Drop ◽  
Vapor Pressure ◽  
Electric Field ◽  
Heat Pipe ◽  
Liquid Velocity ◽  
Thermal Parameters ◽  
Working Fluid ◽  
Liquid Pressure ◽  
Pressure Drops ◽  
Capillary Limit

The present work highlights the impact of the working dielectric fluid on the flow and the thermal parameters of an axially grooved flat mini heat pipe (FMHP) submitted to Electrohydrodynamic (EHD) effects. Three dielectric working fluids are considered: pentane, R123, and R141b. A model is developed by considering the Laplace-Young, mass, momentum, and energy balance equations. The numerical results show that the electric field affects the liquid distribution along the heat pipe and helps the condensate to flow back to the evaporator section. Moreover, under the electric field conditions, the vapor pressure drop increases, however, the liquid pressure drop decreases. The effect of the electric field on the liquid velocity depends on the FMHP zone, and the vapor velocity is hardly affected by the EHD effects. Furthermore, lower capillary driving pressures are required to provide the necessary capillary pumping under EHD conditions. Besides, pentane allows for higher vapor pressure drops compared to those obtained with R123 and R141b, while the liquid pressure drops are highest for R123. It is found that with R123, the liquid velocity is higher than that reached with R141b and pentane. It is also demonstrated that the capillary limit increases under EHD conditions, and for R141b, the capillary limit is the highest either in zero-field and EHD conditions. Best heat pipe thermal performances are observed for wide and deep grooves with R141b. Finally, the optimum fill charge allowing the maximum heat transfer capacity is determined for each working fluid and different groove dimensions. It is shown that the optimum fill charge is hardly affected by the electric field whatever the working fluid. R123 requires the highest optimum fill charge, however, the heat transport capacity of the FMHP is the lowest when using this working fluid.

The Effects of Bending on Heat Pipes

2021 ◽  
Author(s):  
Mahboobe Mahdavi ◽  
Amir Faghri
Keyword(s):  
Pressure Drop ◽  
Heat Pipe ◽  
Three Dimensional ◽  
Heat Pipes ◽  
Operating Pressure ◽  
Liquid Pressure ◽  
Physical Conditions ◽  
Capillary Limit ◽  
Sintered Powder ◽  
Screen Mesh

Abstract A comprehensive three-dimensional numerical model is developed to evaluate the effect of bending on water-copper cylindrical heat pipes. This model distinguishes itself from other models by its ability to uniquely determine the operating pressure of the heat pipe based on the operating and physical conditions. The effects of one 90-degree bend and two 90-degree bends are evaluated on the performance of a heat pipe. Two types of wicks are considered: a screen mesh wick and a sintered powder wick. The obtained results show that bending does affect the vapor pressure drop; however, the changes are not significant when compared to the operating pressure of the heat pipe. If the bending is performed in a manner where the wick is not damaged and the liquid is not blocked from returning to the evaporator, the performance of the heat pipe will not be affected significantly. In addition, if the heat pipe is operating in the horizontal direction, where both evaporator and condenser legs are at the same level, bending does not affect the liquid pressure drop significantly; however, the screen mesh does provide a higher capillary limit. The results also showed that the effects of gravity can be important when bending heat pipes and consideration should be given for this factor. When the bent heat pipe works against gravity, the sintered powder wick heat pipes showed higher capillary limits.

Numerical Study on the Performance Characteristics of Cylindrical Heat Pipes With Differing Wick Type

2018 ◽  
Author(s):  
Mahboobe Mahdavi ◽  
Saeed Tiari ◽  
Ajaysinh Solanki ◽  
Vivek Pawar
Keyword(s):  
Thermal Resistance ◽  
Heat Pipe ◽  
Operating Temperature ◽  
Numerical Study ◽  
Operating Conditions ◽  
Liquid Sodium ◽  
Working Fluid ◽  
Liquid Pressure ◽  
Pressure Drops ◽  
Sintered Powder

In the current study, the performance of a high temperature, cylindrical heat pipe under various operating conditions is investigated numerically. To find the appropriate geometrical and working parameters of the heat pipe, a two-dimensional axisymmetric model is developed to describe the vapor and liquid flows and heat transfers in the vapor core, the wick, and the wall regions. Sodium and stainless steel are selected as the working fluid, the wick material, and the container material. The compressibility of the vapor and viscous dissipation are taken into account. In the wick region, the Darcy–Brinkman–Forchheimer model is applied to simulate the liquid sodium characteristics. The effect of wick type, heat input, and operating temperature are studied on the overall performance of the heat pipe as well as vapor and liquid pressure drops. Screen wick, sintered powder wick and felt wick are selected. The results showed that, for the selected wick types, the sintered powder wick resulted in the largest liquid pressure drop and the felt wick resulted in the lowest thermal resistance. In addition, the influence of operating temperature on thermal resistance diminishes with increasing temperature.

Fluid Flow and Capillary Limit in Heat Pipe Thermal Ground Planes

2013 ◽  
Author(s):  
Mohammed T. Ababneh ◽  
Frank M. Gerner
Keyword(s):  
Shear Stress ◽  
Fluid Flow ◽  
Pressure Drop ◽  
Heat Pipe ◽  
Interfacial Shear Stress ◽  
Working Fluid ◽  
Interfacial Heat Transfer Coefficient ◽  
Evaporation And Condensation ◽  
Capillary Limit ◽  
Energy Balances

This work shows the solution of the fluid flow and the capillary limit in heat pipe thermal ground planes after solving the temperature field. In addition, the effect of wall shear stress and the interfacial shear stress in the liquid pressure of the TGP is studied. In order to obtain more accurate results it is necessary to solve the velocity and thermal fields in both the liquid saturated wick and the vapor. It is also important to account for the mass, momentum and energy balances at the interface between the vapor and liquid. Previous work demonstrated that for the TGP’s which utilize water as the working fluid, the Jacob number is very small. A consequence of this is that convection of liquid with the wick is much smaller than conduction and the temperature may be solved independently of the velocity field. These solutions were presented in previous work. A key feature of the thermal model is that it relies on empirical interfacial heat transfer coefficient data to very accurately model the interfacial energy balance at the vapor-liquid saturated wick interface. One important result uses a solution for the evaporation and condensation rates and hence normal velocities at the interface. The results show that for all of the TGP’s lengths, the ratio between the pressure drop in the vapor and the pressure drop in the liquid is close to zero. Therefore, the pressure drop in the liquid will determine the capillary limit in the TGP.

Operational Limitations of Heat Pipes With Silver-Water Nanofluids

2013 ◽  
Vol 135 (11) ◽  
Author(s):  
Lazarus Godson Asirvatham ◽  
Rajesh Nimmagadda ◽  
Somchai Wongwises
Keyword(s):  
Silver Nanoparticles ◽  
Heat Pipe ◽  
Heat Load ◽  
Operating Temperature ◽  
Particle Diameter ◽  
Heat Pipes ◽  
Working Fluid ◽  
Limit Value ◽  
Capillary Limit ◽  
Nanoparticles Concentration

The paper presents the enhancement in the operational limits (boiling, entrainment, sonic, viscous and capillary limits) of heat pipes using silver nanoparticles dispersed in de-ionized (DI) water. The tested nanoparticles concentration ranged from 0.003 vol. % to 0.009 vol. % with particle diameter of <100 nm. The nanofluid as working fluid enhances the effective thermal conductivity of heat pipe by 40%, 58%, and 70%, respectively, for volume concentrations of 0.003%, 0.006%, and 0.009%. For an input heat load of 60 W, the adiabatic vapor temperatures of nanofluid based heat pipes are reduced by 9 °C, 18 °C, and 20 °C, when compared with DI water. This reduction in the operating temperature enhances the thermophysical properties of working fluid and gives a change in the various operational limits of heat pipes. The use of silver nanoparticles with 0.009 vol. % concentration increases the capillary limit value of heat pipe by 54% when compared with DI water. This in turn improves the performance and operating range of the heat pipe.

Effect of Viscosity on the Pressure Gradient in 4-Inch Pipe

2014 ◽  
Author(s):  
M. Mudasar Imam ◽  
Mehaboob Basha ◽  
S. M. Shaahid ◽  
Aftab Ahmad ◽  
Luai M. Al-Hadhrami
Keyword(s):  
Pressure Drop ◽  
Liquid Velocity ◽  
Pressure Drops ◽  
Increase With Increase ◽  
Empirical Relations ◽  
Inclination Angles ◽  
The Difference ◽  
Reference Pressure ◽  
Velocity Pressure ◽  
Measured Pressure

The pressure drop of liquids of different viscosities in multiphase flow is still a subject of research. This paper presents pressure drop measurements of water and oil single phase flow in horizontal and inclined 4 inch diameter stainless steel pipe at different flow rates. Potable water and Exxol D80 oil were used in the study. Experiments were carried out for different inclination angles including; 0°, 15°, 30° (upward and downward flows). Inlet liquid velocities were varied from 0.4 to 1.2 m/s and reference pressure was set at 1 bar. Water and Oil viscosities are 0.798 Pa.s and 1.56 Pa.s at 30°C, respectively. Pressure drop has been found to increase with increase in liquid velocity. Pressure drop has been observed to increase asymptotically with pipe inclination. Upward flows are associated with high pressure drop as compared to downward flows. The pressure drop of water is greater than that of oil for all inclinations. This difference can be attributed to the difference in fluid viscosities and densities. Measured pressure drops were compared with existing empirical relations and good agreement was noticed.

scholarly journals Структура потока в межлопаточном канале соплового аппарата с поворотной диафрагмой

2021 ◽  
pp. 35-43
Author(s):  
Александр Григорьевич Жирков ◽  
Александр Павлович Усатый ◽  
Елена Петровна Авдеева ◽  
Юрий Иванович Торба
Keyword(s):  
Pressure Drop ◽  
Kinetic Energy ◽  
Energy Loss ◽  
Numerical Study ◽  
Working Fluid ◽  
Relative Pressure ◽  
Pressure Drops ◽  
Vortex Zone ◽  
Flat Flow ◽  
Tube Channel

In the process of developing a numerical study method of a flat flow around a snap line with a rotary diaphragm, calculations were made at various degrees of opening the rotary diaphragm δ and pressure drops on the grille. As a result of calculations, for small degrees, the opening of the rotary diaphragm, complex patterns of the flow were obtained, in the inter-tube channel of the nozzle apparatus. The article presents some results of a numerical study of the supersonic flow in the channel of the nozzle apparatus with the degree of opening the rotary diaphragm δ = (0.15 ÷ 0.3). Modeling and calculating the flow of the working fluid is made using the Fluent software package. The construction of the calculated areas bounded by one inter-tube channel, for varying degrees of opening the diaphragm of the nozzle apparatus. Grids are built for calculated areas. Calculations were carried out for δ = (0.15 ÷ 0.3) and with different degrees of pressure drop on the grille. As a result of the calculations performed, the flow patterns in the inter-tube canal were obtained and behind it, and the distribution of the coefficients of the kinetic energy loss on the lattice front at various degrees of the discovery of the diaphragm at the inlet in the nozzle apparatus. According to the results of the work carried out, the following conclusions can be drawn: the structure of the stream in the inter-tube channel, the nozzle apparatus at small detection of the discovery, is divided into two parts: a supersonic core of the spawth of the blade and a dialing, the vortex zone at the back of the blade; The supersonic thread kernel at certain values of the relative pressure drop on the lattice (or the air flow values through the grid) is separated by shock fronts into several areas; The coefficients of energy loss, for small degrees of discovery, decrease with a decrease in the relative pressure drops (with an increase in the rate of expiration of the flow from the nozzle lattice); The greatest contribution to the magnitude of the loss of kinetic energy is introduced by a vortex zone in the inter-tube channel, and not wave phenomena in the core of the flow; Optimization of the flow part of the nozzle apparatus must be carried out in order to reduce areas with vortex flow. The results obtained in this work will be used to develop a methodology for a numerical study of the spatial flow around the nozzle lattices with rotary diaphragms.

Flow Characteristics and Frictional Pressure Drops of Gas-Liquid Two-Phase Flow in Mini-Micro Pipes and at Vena Contract and Expansion

2007 ◽  
Author(s):  
Hiroyasu Ohtake ◽  
Hideyasu Ohtaki ◽  
Yasuo Koizumi
Keyword(s):  
Pressure Drop ◽  
Flow Pattern ◽  
Two Phase Flow ◽  
Liquid Velocity ◽  
Flow Patterns ◽  
Experimental Results ◽  
Phase Flow ◽  
Two Phase ◽  
Pressure Drops ◽  
Frictional Pressure Drop

The frictional pressure drops and two-phase flow patterns of gas-liquid two-phase flow in mini-micro pipes and at vena contract and expansion were investigated experimentally. Test liquid was water; test gas was argon. The diameter of the test mini-pipe was 0.5, 0.25 and 0.12 mm, respectively. The pressure drop data and the flow pattern were collected over 2.1 < Ug < 92.5 m/s for the superficial gas velocity and 0.03 < Ul < 10 m/s for the superficial liquid velocity. The experimental results show that the flow patterns were slug, churn, ring and annular flows; pure bubbly flow pattern was not observed in a range of the present experimental conditions. The two-phase friction multiplier data for D > 0.5 mm showed to be in good agreement with the conventional correlations. On the other hand, the two-phase friction multiplier data for D < 0.25 mm differed from the calculated values by the conventional correlations. Then, thickness of liquid film around a gas plug and size of gas core were estimated and the effect of frictional pressure drop on channel size was discussed through Knudsen Number of gas and instability on liquid-gas interface. The coefficients of sudden enlargement and sudden contraction in mini-pipes for the gas-water two-phase flow were modified from the present experimental results.

An Experimental Investigation of Condensation Heat Transfer Coefficients and Pressure Drops of Refrigerants Inside Multiport Mini-channel Tubes

2017 ◽  
Vol 25 (02) ◽  
pp. 1750013 ◽  
Author(s):  
Pham-Quang Vu ◽  
Kwang-Il Choi ◽  
Jong-Taek Oh ◽  
Honggi Cho
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Pressure Drop ◽  
Mass Flux ◽  
Condensation Heat Transfer ◽  
Working Fluid ◽  
Vapor Quality ◽  
Transfer Coefficients ◽  
Pressure Drops ◽  
Heat Transfer Coefficients

The condensation heat transfer coefficients and pressure drops of R410A and R22 flowing inside a horizontal aluminum multiport mini-channel tube having 18 channels are investigated. Experimental data are presented for the range of vapor quality from 0.1 to 0.9, mass flux from 50 to 500[Formula: see text]kg/m2s, heat flux from 3 to 15[Formula: see text]kW/m2 and the saturation temperature at 48[Formula: see text]C. The pressure drop across the test section was directly measured by a differential pressure transducer. At a small scale, the noncircular cross-sections can enhance the effect of the surface tension. The average heat transfer coefficient increased with the increase of vapor quality, mass flux and heat flux. Under the same test conditions, the heat transfer coefficients of R22 are higher than those for R410A, the pressure drops for R410A are 7–19% lower than those of R22. The lower pressure drop of R410A has an important advantage as an alternative working fluid for R22 in air-conditioning and heat pump systems.

Competitive Designs of Evaporator Top Cap of the Micro Loop Heat Pipe

2004 ◽  
Author(s):  
Praveen Kumar Arragattu ◽  
Frank M. Gerner ◽  
Priyanka Ponugoti ◽  
H. T. Henderson
Keyword(s):  
Finite Element ◽  
Pressure Drop ◽  
Finite Element Modeling ◽  
Heat Pipe ◽  
Temperature Drop ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Two Phase ◽  
Maximum Heat ◽  
Element Modeling

The Micro Loop Heat Pipe (LHP) is a two phase device that may be used to cool electronics, solar collectors and other devices in space applications. A LHP is a two-phase device with extremely high effective thermal conductivity that utilizes the thermodynamic pressure difference developed between the evaporator and condenser and capillary forces developed inside its wicked evaporator to circulate a working fluid through a closed loop. While previous experiments have shown reduction in chip temperature, maximum heat flux was less than theoretically predicted. This paper addresses the main problem with the past designs of top cap which has been the conduction of heat from the heat source to the primary wick. The new top cap design provides conduction pathways which enables the uniform distribution of heat to the wick. The provision of conduction pathways in the top cap increases the pressure losses and decreases the temperature drop. The feasible competitive designs of the top cap with conduction pathways from the fabrication point of view were discussed in detail. Calculation of pressure drop and temperature drop is essential for the determination of optimal solutions of the top cap. Approximate pressure drop was calculated for the top cap designs using simple 2-D microchannel principles. Finite element modeling was performed to determine the temperature drop in the conduction pathways. The conditions used for arriving at the optimal design solutions are discussed. A trapezoidal slot top cap design was chosen for fabrication as it was relatively easy to fabricate with available MEMS fabrication technologies. The exact pressure drop calculation was performed on the fabricated top cap using commercial flow solver FLUENT 6.1 with appropriate boundary conditions. The temperature drop calculation was performed by finite element modeling in ANSYS 6.1. Obtained values of pressure drop and temperature drop for fabricated trapezoidal slot top cap was found to be within the optimal limits.

The Inlet Air Pre-Cooling of Natural Draft Dry Cooling Towers-Wetted Medium Issue

2017 ◽  
Author(s):  
Suoying He ◽  
Guanhong Zhang ◽  
Yi Xu ◽  
Fengzhong Sun
Keyword(s):  
Pressure Drop ◽  
Ambient Air ◽  
Western China ◽  
Working Fluid ◽  
Cooling Towers ◽  
Pressure Drops ◽  
Natural Draft ◽  
Western Asia ◽  
Cooling Enhancement ◽  
Dry Cooling

Dry cooling towers are an alternative cooling method when large quantities of water are not available. Examples of the proposed applications are the enhanced geothermal and concentrated solar thermal (CST) power plants in arid or semi-arid areas, like south-western United States, Australia, western Asia, north-western China and the rest of the world. Natural draft dry cooling towers (NDDCTs) have received widespread attention because they do not consume water, have low maintenance requirements and cause small parasitic losses. Unfortunately, the performance of a NDDCT is severely reduced when the ambient air is hot, which is because the NDDCT is driven by buoyancy effect and relies solely on air to cool the working fluid. The present study introduces inlet air pre-cooling using wetted media, which combines dry and wet cooling. The wet cooling system only operates at high ambient temperatures to assist dry cooling. However, wetted-medium cooling introduces extra pressure drop which reduces the air flow passing through the NDDCT and thus impairs the tower heat rejection. To this end, this paper takes into account the trade-off between the wetted-medium cooling and the extra pressure drop. Early studies find that the performance of NDDCTs can be improved by wetted-medium evaporative pre-cooling when the ambient air is hot and dry. However, the pre-cooling enhancement is seasonal-dependent and is significantly affected by wetted media. To further investigate the effect of wetted medium type on pre-cooling performance, the current study simulates a pre-cooled NDDCT using five selected wetted media (i.e., three film and two trickle media) based on a self-developed MATLAB program. The innovations of the current study are: (1) two typical types of wetted media with the potential of evaporative pre-cooling are comparatively studied to give suggestions for future pre-cooling design; (2) the characteristics of wetted media suitable for evaporative pre-cooling of NDDCTs are summarized. The simulation finds that the media with high or low cooling efficiencies and pressure drops are not promising while those media with middle cooling efficiencies and pressure drops intend to produce much performance enhancement of the studied NDDCT. The film medium, Cellulose7060 with pressure drops of 28.6–272.1 Pa/m and cooling efficiency range of 44.7–88.5% is most promising for such pre-cooling enhancement. For the studied NDDCT, the critical temperatures below which the tower performance does not benefit but is hindered by wetted-medium pre-cooling are 28, 16, 30, 26 and 26°C for cellulose7090, cellulose7060, PVC1200, Trickle125 and Trickle100, respectively (ambient humidity of 20% and medium thickness of 200mm). The pre-cooling enhancements can go up to 100% by 200mm-thick cellulose7060 at extreme hot and dry climate (i.e., ambient temperature of 50°C and humidity of 20%). The simulation will give instructions for the design of pre-cooled NDDCTs.

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