scholarly journals Finite Element Analysis of Air Flow and Temperature Distribution on Surface of a Circular Obstacle with Resistance and Orientation of Screen

2021 ◽  
Vol 2021 ◽  
pp. 1-12
Author(s):  
Abid A. Memon ◽  
M. Asif Memon ◽  
Aisha M. Alqahtani ◽  
Kaleemullah Bhatti ◽  
Kamsing Nonlaopon ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Finite Element ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Rectangular Channel ◽  
Resistance Coefficient ◽  
Working Fluid ◽  
Nonisothermal Flow ◽  
Circular Surface

Nonisothermal flow through the rectangular channel on a circular surface under the influence of a screen embedded at the middle of a channel at angles θ is considered. Simulations are carried out via COMSOL Multiphysics 5.4 which implements the finite element method with an emerging technique of the least square procedure of Galerkin’s method. Air as working fluid depends upon the Reynolds number with initial temperature allowed to enter from the inlet of the channel. The nonisothermal flow has been checked with the help of parameters such as Reynolds number, angle of the screen, and variations in resistance coefficient. The consequence and the pattern of the velocity field, pressure, temperature, heat transfer coefficient, and local Nusselt number are described on the front surface of the circular obstacle. The rise in the temperature and the flow rate on the surface of the obstacle has been determined against increasing Reynolds number. Results show that the velocity magnitudes are decreasing down the surface and the pressure is increasing down the surface of the obstacle. The pressure on the surface of the circular obstacle was found to be the function of the y-axis and does not show any impact due to the change of the resistance coefficient. Also, it was indicated that the temperature on the front circular surface does not depend upon the orientation of the screen and resistance factor. The heat transfer coefficient is decreasing which indicates that the conduction process is dominating over the convection process.

Computer Simulation of Mixed Convection of Alumina-Deionized Water Nanofluid Over Four In-Line Electronic Chips Embedded in One Wall of a Vertical Rectangular Channel

2019 ◽  
Vol 12 (4) ◽  
Author(s):  
Nalla Ramu ◽  
P. S. Ghoshdastidar
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Mixed Convection ◽  
Transfer Coefficient ◽  
Base Fluid ◽  
Rectangular Channel ◽  
Working Fluid ◽  
Al2o3 Nanoparticles ◽  
Enhancement Factors

Abstract This paper presents a computational study of mixed convection cooling of four in-line electronic chips by alumina-deionized (DI) water nanofluid. The chips are flush-mounted in the substrate of one wall of a vertical rectangular channel. The working fluid enters from the bottom with uniform velocity and temperature and exits from the top after becoming fully developed. The nanofluid properties are obtained from the past experimental studies. The nanofluid performance is estimated by computing the enhancement factor which is the ratio of chips averaged heat transfer coefficient in nanofluid to that in base fluid. An exhaustive parametric study is performed to evaluate the dependence of nanoparticle volume fraction, diameter of Al2O3 nanoparticles in the range of 13–87.5 nm, Reynolds number, inlet velocity, chip heat flux, and mass flowrate on enhancement in heat transfer coefficient. It is found that nanofluids with smaller particle diameters have higher enhancement factors. It is also observed that enhancement factors are higher when the nanofluid Reynolds number is kept equal to that of the base fluid as compared with the cases of equal inlet velocities and equal mass flowrates. The linear variation in mean pressure along the channel is observed and is higher for smaller nanoparticle diameters.

Turbulent Heat Transfer in the Separated, Reattached, and Redevelopment Regions of a Circular Tube

1966 ◽  
Vol 88 (1) ◽  
pp. 131-136 ◽  
Author(s):  
K. M. Krall ◽  
E. M. Sparrow
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flow Separation ◽  
Circular Tube ◽  
Turbulent Pipe Flow ◽  
Turbulent Heat Transfer ◽  
Working Fluid ◽  
The Heat Transfer Coefficient

Experiments were performed to determine the effect of flow separation on the heat-transfer characteristics of a turbulent pipe flow. The flow separation was induced by an orifice situated at the inlet of an electrically heated circular tube. The degree of flow separation was varied by employing orifices of various bore diameters. Water was the working fluid. The Reynolds number and the Prandtl number, respectively, ranged from 10,000 to 130,000 and from 3 to 6. The measurements show that the local heat-transfer coefficients in the separated, reattached, and redevelopment regions are several times as large as those for a fully developed flow. For instance, at the point of reattachment, the coefficients were 3 to 9 times greater than the corresponding fully developed values. In general, the increase of the heat-transfer coefficient owing to flow separation is accentuated as the Reynolds number decreases. The point of flow reattachment, which corresponds to a maximum in the distribution of the heat-transfer coefficient, was found to occur from 1.25 to 2.5 pipe dia from the onset of separation.

CFD Study of the Thermal Performance Improvement of a Counter-Flow Heat Exchanger Using Enhanced Surface Tubes

2021 ◽  
Author(s):  
Humberto Santos ◽  
Wei Li ◽  
David Kukulka
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Thermal Performance ◽  
Working Fluid ◽  
Operational Parameters ◽  
Ansys Fluent ◽  
Flow Heat ◽  
The Heat Transfer Coefficient

Abstract A CFD investigation was carried out to compare the thermal performance of the 1EHT-1 and 1EHT-2 tubes with a smooth surface tube using R410A at 311K as working fluid. These tubes have enhanced heat transfer area generated by a series of dimples/protrusions and petals distributed over its surface. All the stages of this simulation were conducted using Ansys Fluent. Initially, the physical model of the fluid domain was developed using the Design Modeler module, with an internal tube diameter of 8.32mm, and then imported to the meshing module for the griding process. To ensure accuracy in the results, the mesh average orthogonal quality was kept above 0.7, with the minimum orthogonal quality higher than 0.1. For the numerical simulation, SST k-omega model was used, with Reynolds number ranging from 16000 to 35000. The results of the heat transfer coefficient were validated based on previous experimental work. As expected, at the lowest Reynolds number tested, the heat transfer coefficient for the 1EHT-1 tube was 1097.5 W.K−1.m−2, followed by 1058 W.K−1.m−2 for the 1EHT-2 and nearly 846 W.K−1.m−2 for the smooth tube. When compared with the experimental results, a good agreement was observed, and the HTC relative error (RE) for all tubes tested was below 10%. It is possible to conclude that the CFD model used here presents as powerful tool to simulate and predict heat transfer with good accuracy, allowing optimization in heat exchangers design and operational parameters.

Confined Jet Impingement Thermal Management Using Liquid Ammonia as the Working Fluid

2002 ◽  
Author(s):  
Muhammad M. Rahman ◽  
Padmaja Dontaraju ◽  
Rengasamy Ponnappan
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Numerical Models ◽  
Strong Dependence ◽  
Working Fluid ◽  
Interface Temperature ◽  
Fluid Interface ◽  
First Case

The focus of the study was the conjugate heat transfer during impingement of a confined liquid jet. Two numerical models of a heat transfer process with heat transmission through a fluid-solid interface have been developed. In the first case only the fluid region has been considered while in the second case the solid region has been modeled along with the fluid region as a conjugate problem. The inlet nozzle Reynolds number has been kept at values where laminar flow can be assumed in all cases. The solid-fluid interface temperature shows a strong dependence on several geometric, fluid flow, and heat transfer parameters. The Nusselt number increased with Reynolds number. For a given flow rate, a higher heat transfer coefficient was obtained with smaller slot width and lower impingement height. A higher heat transfer coefficient at the impingement location was seen at a smaller thickness, whereas a thicker plate provided a more uniform distribution of heat transfer coefficient. Compared to Mil-7808 and FC-77, ammonia provided much smaller solid-fluid interface temperature and higher heat transfer coefficient.

Flow and Thermal Fields in a Pendant Droplet Moving on Lyophobic Surface

2010 ◽  
Author(s):  
Basant Singh Sikarwar ◽  
K. Muralidhar ◽  
Sameer Khandekar
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Reynolds Number ◽  
Shear Stress ◽  
Heat Transfer Coefficient ◽  
Wall Shear Stress ◽  
Transfer Coefficient ◽  
Maximum Shear Stress ◽  
Computational Domain ◽  
Wall Shear

Clusters of liquid drops growing and moving on physically or chemically textured lyophobic surfaces are encountered in drop-wise mode of vapor condensation. As opposed to film-wise condensation, drops permit a large heat transfer coefficient and are hence attractive. However, the temporal sustainability of drop formation on a surface is a challenging task, primarily because the sliding drops eventually leach away the lyophobicity promoter layer. Assuming that there is no chemical reaction between the promoter and the condensing liquid, the wall shear stress (viscous resistance) is the prime parameter for controlling physical leaching. The dynamic shape of individual droplets, as they form and roll/slide on such surfaces, determines the effective shear interaction at the wall. Given a shear stress distribution of an individual droplet, the net effect of droplet ensemble can be determined using the time averaged population density during condensation. In this paper, we solve the Navier-Stokes and the energy equation in three-dimensions on an unstructured tetrahedral grid representing the computational domain corresponding to an isolated pendant droplet sliding on a lyophobic substrate. We correlate the droplet Reynolds number (Re = 10–500, based on droplet hydraulic diameter), contact angle and shape of droplet with wall shear stress and heat transfer coefficient. The simulations presented here are for Prandtl Number (Pr) = 5.8. We see that, both Poiseuille number (Po) and Nusselt number (Nu), increase with increasing the droplet Reynolds number. The maximum shear stress as well as heat transfer occurs at the droplet corners. For a given droplet volume, increasing contact angle decreases the transport coefficients.

A Loop Thermosyphon With Hydrophobic Spots Evaporator Surface

2018 ◽  
Author(s):  
Hongbin He ◽  
Biao Shen ◽  
Sumitomo Hidaka ◽  
Koji Takahashi ◽  
Yasuyuki Takata
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Nucleate Boiling ◽  
Working Fluid ◽  
Immersion Method ◽  
Two Phase ◽  
Coating Materials ◽  
High Heat Transfer ◽  
Coated Surface

Heat transfer characteristic of a closed two-phase thermosyphon with enhanced boiling surface is studied and compared with that of a copper mirror surface. Two-phase cooling improves heat transfer coefficient (HTC) a lot compared to single-phase liquid cooling. The evaporator surfaces, coated with a pattern of hydrophobic circle spots (non-electroplating Ni-PTFE, 0.5∼2 mm in diameter and 1.5–3 mm in pitch) on Cu substrates, achieve very high heat transfer coefficient and lower the incipience temperature overshoot using water as the working fluid. Sub-atmospheric boiling on the hydrophobic spot-coated surface shows a much better heat transfer performance. Tests with heat loads (30 W to 260 W) reveals the coated surfaces enhance nucleate boiling performance by increasing the bubbles nucleation sites density. Hydrophobic circle spots coated surface with diameter 1 mm, pitch 1.5 mm achieves the maximal heat transfer enhancement with the minimum boiling thermal resistance as low as 0.03 K/W. The comparison of three evaporator surfaces with same spot parameters but different coating materials is carried out experimentally. Ni-PTFE coated surface with immersion method performs the optimal performance of the thermosyphon.

A Fundamental View of the Flow Boiling Heat Transfer Characteristics of Nano-Refrigerants

2012 ◽  
Author(s):  
Lorenzo Cremaschi
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Liquid Phase ◽  
Transfer Coefficient ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Working Fluid ◽  
Heat Transfer Characteristics ◽  
Transfer Characteristics ◽  
Flow Boiling Heat Transfer

Driven by higher energy efficiency targets and industrial needs of process intensification and miniaturization, nanofluids have been proposed in energy conversion, power generation, chemical, electronic cooling, biological, and environmental systems. In space conditioning and in cooling systems for high power density electronics, vapor compression cycles provide cooling. The working fluid is a refrigerant and oil mixture. A small amount of lubricating oil is needed to lubricate and to seal the sliding parts of the compressors. In heat exchangers the oil in excess penalizes the heat transfer and increases the flow losses: both effects are highly undesired but yet unavoidable. This paper studies the heat transfer characteristics of nanorefrigerants, a new class of nanofluids defined as refrigerant and lubricant mixtures in which nano-size particles are dispersed in the high-viscosity liquid phase. The heat transfer coefficient is strongly governed by the viscous film excess layer that resides at the wall surface. In the state-of-the-art knowledge, while nanoparticles in the refrigerant and lubricant mixtures were recently experimentally studied and yielded convective in-tube flow boiling heat transfer enhancements by as much as 101%, the interactions of nanoparticles with the mixture still pose several open questions. The model developed in this work suggested that the nanoparticles in this excess layer generate a micro-convective mass flux transverse to the flow direction that augments the thermal energy transport within the oil film in addition to the macroscopic heat conduction and fluid convection effects. The nanoparticles motion in the shearing-induced and non-uniform shear rate field is added to the motion of the nanoparticles due to their own Brownian diffusion. The augmentation of the liquid phase thermal conductivity was predicted by the developed model but alone it did not fully explain the intensification on the two-phase flow boiling heat transfer coefficient reported in previous work in the literature. Thus, additional nano- and micro-scale heat transfer intensification mechanisms were proposed.

scholarly journals Toward improved heat dissipation of the turbulent regime over backward-facing step for the AL2O3-water nanofluids: An experimental approach

2019 ◽  
Vol 23 (3 Part B) ◽  
pp. 1779-1789 ◽  
Author(s):  
Syed Ahmed ◽  
Salim Kazi ◽  
Ghulamullah Khan ◽  
Mohd Zubir ◽  
Mahidzal Dahari ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Experimental Study ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Turbulent Regime ◽  
Al2o3 Nanoparticles ◽  
Backward Facing Step ◽  
Constant Weight ◽  
Weight Concentration

Experimental study of nanofluid flow and heat transfer to fully developed turbulent forced convection flow in a uniformly heated tubular horizontal backward-facing step has reported in the present study. To study the forced convective heat transfer coefficient in the turbulent regime, an experimental study is performed at a different weight concentration of Al2O3 nanoparticles. The experiment had conducted for water and Al2O3 -water nanofluid for the concentration range of 0 to 0.1 wt.% and Reynolds number of 4000 to 16000. The average heat transfer coefficient ratio increases significantly as Reynolds number increasing, increased from 9.6% at Reynolds number of 4000 to 26.3% at Reynolds number of 16000 at the constant weight concentration of 0.1%. The Al2O3 water nanofluid exhibited excellent thermal performance in the tube with a backwardfacing step in comparison to distilled water. However, the pressure losses increased with the increase of the Reynolds number and/or the weight concentrations, but the enhancement rates were insignificant.

The Simulation Study of Turbulence Models for Conjugate Heat Transfer Analysis of a High Pressure Air-Cooled Gas Turbine

2010 ◽  
Author(s):  
Zhenfeng Wang ◽  
Peigang Yan ◽  
Hongfei Tang ◽  
Hongyan Huang ◽  
Wanjin Han
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Gas Turbine ◽  
Turbulence Model ◽  
Conjugate Heat Transfer ◽  
Turbulence Models ◽  
Turbulence Characteristic ◽  
The Heat Transfer Coefficient

The different turbulence models are adopted to simulate NASA-MarkII high pressure air-cooled gas turbine. The experimental work condition is Run 5411. The paper researches that the effect of different turbulence models for the flow and heat transfer characteristics of turbine. The turbulence models include: the laminar turbulence model, high Reynolds number k-ε turbulence model, low Reynolds number turbulence model (k-ω standard format, k-ω-SST and k-ω-SST-γ-θ) and B-L algebra turbulence model which is adopted by the compiled code. The results show that the different turbulence models can give good flow characteristics results of turbine, but the heat transfer characteristics results are different. Comparing to the experimental results, k-ω-SST-θ-γ turbulence model results are more accurate and can simulate accurately the flow and heat transfer characteristics of turbine with transition flow characteristics. But k-ω-SST-γ-θ turbulence model overestimates the turbulence kinetic energy of blade local region and makes the heat transfer coefficient higher. It causes that local region temperature is higher. The results of B-L algebra turbulence model show that the results of B-L model are accurate besides it has 4% temperature error in the transition region. As to the other turbulence models, the results show that all turbulence models can simulate the temperature distribution on the blade pressure surface except the laminar turbulence model underestimates the heat transfer coefficient of turbulence flow region. On the blade suction surface with transition flow characteristics, high Reynolds number k-ε turbulence model overestimates the heat transfer coefficient and causes the blade surface temperature is high about 90K than the experimental result. Low Reynolds number k-ω standard format and k-ω-SST turbulence models also overestimate the blade surface temperature value. So it can draw a conclusion that the unreasonable choice of turbulence models can cause biggish errors for conjugate heat transfer problem of turbine. The combination of k-ω-SST-γ-θ model and B-L algebra model can get more accurate turbine thermal environment results. In addition, in order to obtain the affect of different turbulence models for gas turbine conjugate heat transfer problem. The different turbulence models are adopted to simulate the different computation mesh domains (First case and Second case). As to each cooling passages, the first case gives the wall heat transfer coefficient of each cooling passages and the second case considers the conjugate heat transfer course between the cooling passages and blade. It can draw a conclusion that the application of heat transfer coefficient on the wall of each cooling passages avoids the accumulative error. So, for the turbine vane geometry models with complex cooling passages or holes, the choice of turbulence models and the analysis of different mesh domains are important. At last, different turbulence characteristic boundary conditions of turbine inner-cooling passages are given and K-ω-SST-γ-θ turbulence model is adopted in order to obtain the effect of turbulence characteristic boundary conditions for the conjugate heat transfer computation results. The results show that the turbulence characteristic boundary conditions of turbine inner-cooling passages have a great effect on the conjugate heat transfer results of high pressure gas turbine.

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