Numerical study on evaporation heat transfer characteristics of water in inclined microchannels with varying inlet vapor quality

2019 ◽  
Vol 16 (1) ◽  
pp. 125-131
Author(s):  
Vivekanand SVB ◽  
Raju VRK
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Vapor Phase ◽  
Heated Wall ◽  
Phase Flow ◽  
Vapor Quality ◽  
Heat Transfer Characteristics ◽  
Two Phase ◽  
Ansys Fluent ◽  
Content Type

PurposeThe purpose of this paper is to investigate the effects of gravity on the heat transfer behavior of the two-phase flow of water undergoing phase change. Most of the earlier studies of convective boiling considered systems where the gravity is neglected. In contrast, the authors investigated systems where the gravity is considered. The heat transfer characteristics of water during its evaporation in microchannel heat sink are studied for different channel inclinations.Design/methodology/approachComputational fluid dynamics software ANSYS Fluent is used for the computational study. The volume of fluids multiphase method available in the package is used to capture the vapor–liquid interface. Heat transfer studies are carried out for a rectangular microchannel having a characteristic dimension of 825 µm at different inclinations, which varied from −90° (vertically downward) to 90° (vertically upward). During each simulation, the vapor quality is set at the inlet. Uniform heat flux of 250 kW/m2is applied at the bottom wall of the channel in all orientations of the channel, keeping the upper wall insulated.FindingsAs compared to horizontal configuration, a significant increase in the values of heat transfer coefficient during the fluid flow in inclined microchannels is noticed. It is observed that the Nusselt number for the vertically upward (+90°) and horizontal (0°) configuration are similar and that for the 45° upward configuration exceeds other configurations. It is also observed that the heat transfer performance becomes lower in downward configurations; nearly 40-50 per cent drop in average Nusselt number is observed for a mass flux of 250 kg m-2s-1with respect to 45° inclined microchannel. This behavior can be attributed to the gravitational effect on the two-phase flow because of which the vapor phase being less dense moves away from the heated wall, whereas the primary phase being heavier moves towards the heated wall of the channel. Also, the conductivity of the liquid being higher than the vapor phase, as well as the aperture of the liquid being small during this process, its velocity increases resulting in the augmentation of heat transfer.Originality/valueUser-defined-functions for the mass and energy source terms have been written in C code and hooked in ANSYS Fluent to incorporate the phase change mechanism during the evaporation of water.

The Effect of Channel Diameter on Heat Transfer Characteristics of Two-Phase Flow in Microchannels

2011 ◽  
Author(s):  
Kyosung Choo ◽  
Daniel Trainer ◽  
Sung Jin Kim
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Gas Flow ◽  
Two Phase Flow ◽  
Superficial Velocity ◽  
Phase Flow ◽  
Heat Transfer Characteristics ◽  
Two Phase ◽  
Channel Diameter ◽  
Transfer Characteristics

The heat transfer and fluid flow characteristics of non-boiling two-phase flow in microchannels were experimentally investigated. The effects of channel diameter (140, 222, 334, and 506 μm) on the Nusselt number were considered. Air and water were used as the working fluids. Results were presented for the Nusselt number over a wide range of gas superficial velocity (1.24–40.1 m/s), liquid superficial velocity (0.57–2.13 m/s), and wall heat flux (0.34–0.95 MW/m2). The results showed that the Nusselt number increased with increasing gas flow rate for the 506 μm and 334 μm channels, while the Nusselt number decreased with increasing gas flow for the 222 μm and 140 μm channels. Based on these experimental results, a transition channel diameter of about 235 μm to 260 μm, which distinguishes microchannels from minichannels, was suggested. By observing two-phase flow patterns within the microchannels, viscosity and surface tension were identified as the key factors that caused the heat transfer characteristics to change. In addition, new correlations for the forced convection Nusselt number were developed.

Heat Transfer characteristics of Mixed Convective Magnetohydrodynamic Counter-Current Two-Phase Flow in Rotating Inclined Micro-Porous Channel with Slip Velocity and Asymmetric Thermal Boundary Conditions Using LTNE Model

2021 ◽  
pp. 1-16
Author(s):  
H.P. Rani ◽  
V Leela ◽  
Pulla Nagabhushanam ◽  
R Gangadhara Reddy
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Boundary Conditions ◽  
Wall Temperature ◽  
Slip Velocity ◽  
Phase Flow ◽  
Control Parameters ◽  
Heat Transfer Characteristics ◽  
Two Phase ◽  
Thermal Boundary Conditions

Abstract The heat transfer characteristics of mixed convective two-phase flow in an inclined rotating micro-porous channel kept in a transverse magnetic field are investigated numerically. The counterflow arrangement is assumed within the channel. Slip velocity and asymmetric thermal boundary conditions are assumed. The governing energy equation involves the local thermal non-equilibrium (LTNE) between the two phases. The LTNE implications of control parameters on the flow field variables and the average Nusselt number, Nu, are highlighted and pertinent observations are documented. When confined to a few specific cases, the current results are consistent with previous research work. The effect of inclination angle on fluid velocity is determined by the wall temperature difference ratio. According to the findings, for certain values of the wall temperature differential ratio, the velocity increases with the angle, however it takes on a dual character for other values. The Nusselt number (Nu) is expected to increase with the Biot number, Hartmann number, and rotation parameter, while Nu decreases as the Knudsen number increases. The results show that as the wall temperature ratio increases, the Nu converges to a common minimum value. The database was generated from the validated CFD model covering a range of control parameters arising in the system. The multilayer perceptron (MLP) networks were trained using this CFD dataset to predict Nu. The average relative error in Nu's prediction is found to be ±2%.

scholarly journals Convective Heat Transfer of Ethanol/Polyalphaolefin Nanoemulsion in Mini- and Microchannel Heat Exchangers for High Heat Flux Electronics Cooling

2021 ◽  
Author(s):  
Jaime Rios ◽  
Mehdi Kabirnajafi ◽  
Takele Gameda ◽  
Raid Mohammed ◽  
Jiajun Xu
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Convective Heat ◽  
Phase Flow ◽  
Transfer Performance ◽  
Transfer Enhancement ◽  
Heat Transfer Characteristics ◽  
Two Phase ◽  
Flow And Heat Transfer

The present study experimentally and numerically investigates the flow and heat transfer characteristics of a novel nanostructured heat transfer fluid, namely, ethanol/polyalphaolefin nanoemulsion, inside a conventionally manufactured minichannel of circular cross section and a microchannel heat exchanger of rectangular cross section manufactured additively using the Direct Metal Laser Sintering (DMLS) process. The experiments were conducted for single-phase flow of pure polyalphaolefin (PAO) and ethanol/PAO nanoemulsion fluids with two ethanol concentrations of 4 wt% and 8 wt% as well as for two-phase flow boiling of nanoemulsion fluids to study the effect of ethanol nanodroplets on the convective flow and heat transfer characteristics. Furthermore, the effects of flow regime of the working fluids on the heat transfer performance for both the minichannel and microchannel heat exchangers were examined within the laminar and transitional flow regimes. It was found that the ethanol/PAO nanoemulsion fluids can improve convective heat transfer compared to that of the pure PAO base fluid under both single- and two-phase flow regimes. While the concentration of nanoemulsion fluids did not reflect a remarkable distinction in single-phase heat transfer performance within the laminar regime, a significant heat transfer enhancement was observed using the nanoemulsion fluids upon entering the transitional flow regime. The heat transfer enhancement at higher concentrations of nanoemulsion within the transitional regime is mainly attributed to the enhanced interaction and interfacial thermal transport between ethanol nanodroplets and PAO base fluid. For two-phase flow boiling, heat transfer coefficients of ethanol/PAO nanoemulsion fluids were further enhanced when the ethanol nanodroplets underwent phase change. A comparative study on the flow and heat transfer characteristics was also implemented between the traditionally fabricated minichannel and additively manufactured microchannel of similar dimensions using the same working fluid of pure PAO and the same operating conditions. The results revealed that although the DMLS fabricated microchannel posed a higher pressure loss, a substantial heat transfer enhancement was achieved as compared to the minichannel heat exchanger tested under the same conditions. The non-post processed surface of the DMLS manufactured microchannel is likely to be the main contributor to the augmented heat transfer performance. Further studies are required to fully appreciate the possible mechanisms behind this phenomenon as well as the convective heat transfer properties of nanoemulsion fluids.

Numerical Research on Heat Transfer Characteristics Analysis of Two Particles in Supercritical Water

2021 ◽  
Author(s):  
Xiaoyu Li ◽  
Zhenqun Wu ◽  
Huibo Wang ◽  
Hui Jin
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Phase Flow ◽  
Heat Transfer Characteristics ◽  
Particle Cluster ◽  
Two Phase ◽  
Transfer Characteristics

Abstract In the supercritical water (SCW)-particle two-phase flow of fluidized bed, the particles that make up the particle cluster interact with each other through fluid, and it will affect the flow and heat transfer. However, due to the complex properties of SCW, the research on particle cluster is lacking, especially in terms of heat transfer. This research takes two particles as an example to study the heat transfer characteristics between SCW and another particle when one particle exists. This research uses the distance and angle between the two particles as the influencing factors to study the average heat transfer rate and local heat transfer rate. In this research, it is found that the effect is obvious when L/D = 1.1. When L = 1.1D, the temperature field and the flow field will partially overlap. The overlap of the temperature field will weaken the heat transfer between SCW and the particle. The overlap of the flow field has an enhanced effect on the heat transfer between SCW and the particle. The heat transfer between SCW and particles is simultaneously affected by these two effects, especially local heat transfer rate. In addition, this research also found that as the SCW temperature decreases, the thermal conductivity and specific heat of SCW increases, which enhances the heat transfer between SCW and the particles. This research is of great significance for studying the heat transfer characteristics of SCW-particle two-phase flow in fluidized bed.

Numerical simulation of water/alumina nanofluid mixed convection in square lid-driven cavity

2019 ◽  
Vol 30 (5) ◽  
pp. 2781-2807
Author(s):  
Davood Toghraie ◽  
Ehsan Shirani
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Mixed Convection ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Richardson Number ◽  
Hartmann Number ◽  
Two Phase ◽  
Content Type ◽  
Driven Cavity

Purpose The purpose of this paper is to investigate the mixed convection of a two-phase water–aluminum oxide nanofluid in a cavity under a uniform magnetic field. Design/methodology/approach The upper wall of the cavity is cold and the lower wall is warm. The effects of different values of Richardson number, Hartmann number, cavitation length and solid nanoparticles concentration on the flow and temperature field and heat transfer rate were evaluated. In this paper, the heat flux was assumed to be constant of 10 (W/m2) and the Reynolds number was assumed to be constant of 300 and the Hartmann number and the volume fraction of solid nanoparticles varied from 0 to 60 and 0 to 0.06, respectively. The Richardson number was considered to be 0.1, 1 and 5. Aspect ratios were 1, 1.5 and 2. Findings Comparison of the results of this paper with the results of the numerical and experimental studies of other researchers showed a good correlation. The results were presented in the form of velocity and temperature profiles, stream and isotherm lines and Nusselt numbers. The results showed that by increasing the Hartmann number, the heat transfer rate decreases. An increase from 0 to 20 in Hartmann number results in a 20 per cent decrease in Nusselt numbers, and by increasing the Hartmann number from 20 to 40, a 16 per cent decrease is observed in Nusselt number. Accordingly, it is inferred that by increasing the Hartmann number, the reduction in the Nusselt number is decreased. As the Richardson number increased, the heat transfer rate and, consequently, the Nusselt number increased. Therefore, an increase in the Richardson number results in an increase of the Nusselt number, that is, an increase in Richardson number from 0.1 to 1 and from 1 to 5 results in 37 and 47 per cent increase in Nusselt number, respectively. Originality/value Even though there have been numerous investigations conducted on convection in cavities under various configurations and boundary conditions, relatively few studies are conducted for the case of nanofluid mixed convection in square lid-driven cavity under the effect of magnetic field using two-phase model.

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