CFD modelling of a two-phase closed thermosyphon charged with R134a and R404a

This paper examines the application of CFD modelling to simulate the two-phase heat transfer mechanisms in a wickless heat pipe, also called a thermosyphon. Two refrigerants, R134a and R404a, were selected as the working fluids of the investigated thermosyphon. A CFD model was built to simulate the details of the two-phase flow and heat transfer phenomena during the start-up and steady-state operation of the thermosyphon. The CFD simulation results were compared with experimental measurements, with good agreement obtained between predicted temperature profiles and experimental temperature data, thus confirming that the CFD model was successful in reproducing the heat and mass transfer processes in the R134a and R404a charged thermosyphon, including the pool boiling in the evaporator section and the liquid film in the condenser section

Modelling of the thermal behaviour of a two-phase closed thermosyphon

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonInterest in the use of heat pipe technology for heat recovery and energy saving in a vast range of engineering applications has been on the rise in recent years. Heat pipes are playing a more important role in many industrial applications, especially in increasing energy savings in commercial applications and improving the thermal performance of heat exchangers. Computational techniques play an important role in solving complex flow problems for a large number of engineering applications due to their universality, flexibility, accuracy and efficiency. However, up to now, computational studies on heat pipes are still at an early stage due to the complexity of multiphase flow characteristics and heat and mass transfer phase changes. Therefore, the main objective of this study is to develop a CFD modelling that includes the complex physical phenomena of both the heat transfer processes of evaporation and condensation and the mass transfer process of phase change during the pool boiling and film condensation. In this thesis, two novel numerical models were developed in ANSYS FLUENT. In the first, a two-dimensional CFD model was developed to visualise the two-phase flow and the evaporation, condensation and heat transfer phenomena during the operation of a wickless heat pipe, that otherwise could not be visualised by empirical or experimental work. An in-house code was developed using user-defined functions (UDFs) to enhance the ability of FLUENT to simulate the phase change occurring inside the heat pipe. Three different fluids, water, R134a and R404a, were selected as the working fluids of the investigated wickless heat pipe. The cooling system of the condenser section was simulated separately as a three-dimensional CFD model of a parallel-flow double pipe heat exchanger to model the heat transfer across the condenser section's heat exchanger and predict the heat transfer coefficients. The overall effective thermal resistance along with the temperature profile along the wickless heat pipe have been investigated. An experimental apparatus was built to carry out a thermal performance investigation on a typical wickless heat pipe for the purpose of validating the CFD simulation. A theoretical model based on empirical correlations was developed to predict the heat transfer thermal resistances in the evaporator and the condenser section. The second model was developed to combine the two-dimensional CFD simulation of the wickless heat pipe and the three-dimensional CFD simulation of the condenser section's heat exchanger to simulate the two-phase flow phenomena of boiling and condensation and the cooling system of the condenser section through a comprehensive three-dimensional CFD model of a wickless heat pipe. Two fluids, water and R134a, were selected as the working fluids of the investigated wickless heat pipe. This model was validated using a transparent glass wickless heat pipe to visualise the phenomena of pool boiling and comparing the results with the three-dimensional CFD flow visualisation. This study demonstrated that the proposed CFD models of a wickless heat pipe can successfully reproduce the complex physical phenomena of both the heat transfer process of evaporation and condensation and the mass transfer process of phase change during the pool boiling that takes place in the evaporator section and the filmwise condensation that takes place in the condenser section. The CFD simulation was successful in modelling and visualising the multiphase flow characteristics for water, R134a and R404a, emphasising the difference in pool boiling behaviour between these working fluids. The CFD simulation results were compared with experimental measurements, with good agreement obtained between predicted temperature profiles and experimental temperature data

Mathematical model for graphene nanofluid flow over a stretching surface with velocity slips thermal convective and mass flux conditions

Nowadays graphene is emerging as one of the most exciting nanomaterial due to its continuous 29 electrically conducting behavior even at zero carrier concentration. With this initiation, we investigate the flow of magnetohydrodynamic (MHD) water, water−30%EG, water−50%EG, graphene nanofluid over a stretched surface with thermal convection, and zero mass flux conditions and velocity slips comprising motile microorganisms and nanoparticles. Thermal radiation and Arrhenius activation energy are also be under consideration. The governing fluid equations are solved by Homotopy analysis method (HAM) and computed numerically with shooting technique after employing appropriate transformations. The consequence of numerous physical parameters on velocity, concentration, temperature, and density of motile microorganisms graphs as well as table are used for ethylene glycol based and water-based graphene nanoparticles. Additionally, numerically analyze the designed skin friction, Nusselt number, Sherwood number, and density of motile microorganisms. It is observed that due to heat generation and temperature the improvement of the nonlinear convection variable improves the wall friction. It is also originate that increasing the volume fraction of nanoparticles effectively boosting the thermal conductivity of water−50%EG when compared with water−30%EG and water nanofluids. Ethylene glycol based graphene nanofluids take less time for process as compared to water based nanofluids

Impact of suction with nanoparticles aggregation and joule heating on unsteady MHD stagnation point flow of nanofluids over horizontal cylinder

Significance of study: Nanofluids with aggregation effects mediated by nanoparticles, like geothermal panels and crossflow heat exchangers, ignite new industrial interests. Polymer and conversion processes have transport phenomena in the stagnation zone that must be continuously improved to raise the process quality standard. Aim of study: Hence, the current computational study examines a TiO2−C2H6O2 nanofluid's unsteady stagnation-point flow performance via a shrinking horizontal cylinder. In addition, the effects of a magnetic field, joule-heating viscous dissipation, nanoparticles aggregation and mass suction on the boundary layer flow are reflected. Method: ology: The RK-IV with shooting method is applied to resolve the simplified mathematical model numerically in computing software MATHEMATICA. In certain circumstances, comparing the current and prior findings indicates good agreement with a relative error of around 0%. Findings: The implementation of a heat transfer operation may be improved by increasing suction settings. Unsteadiness, nanoparticle volume fraction, magnetic, curvature, and Eckert number (implies the operating Joule heating and viscous dissipation) all influence heat transfer rate. The velocity and temperature profiles both increase as the unsteadiness, magnetic field, and nanoparticle volume fraction parameters increase, whereas the curvature and suction parameters show the opposite behavior. When the values of the suction parameters were changed from 2.0 to 2.5 with φ = 0.01, the heat transfer rates rose by 4.751%. A comparison shows that the model with aggregation has a better velocity profile, while the model without aggregation has a better temperature profile

Magneto-bioconvection flow of Casson nanofluid configured by a rotating disk in the presence of gyrotatic microorganisms and Joule heating

In this article, we investigate the bioconvection flow of Casson nanofluid by a rotating disk under the impacts of Joule heating, convective conditions, heat source/sink and gyrotactic microorganisms. When Brownian diffusion and thermophoretic effects exist, the Casson fluid is examined. The existing physical problem of Casson nanofluid flow with energy transports is demonstrated under the above considerations in the form of partial differential equations (PDEs). Using the appropriate transformations, the PDEs are converted into non-linear ordinary differential equations (ODEs). The mathematical results are calculated through MATLAB by using the function bvp4c. The problem's results are rigorously examined graphically and described with physical justifications. Velocity fields decrease as the bioconvection Rayleigh parameter rises. The thermal profile and soluteal field of species also magnify with an upsurge in thermophoresis number estimations. The microorganism's fields are decayed by larger microbes Biot number

Mathematical analysis of mixed convective stagnation point flow over extendable porous riga plate with aggregation and joule heating effects

It is still not quite apparent how suspended nanoparticles improve heat transmission. Multiple investigations have demonstrated that the aggregation of nanoparticles is a critical step in improving the thermal conductivity of nanofluids. However, the thermal conductivity of the nanofluid would be greatly affected by the fractal dimension of the nanoparticle aggregation. The purpose of this research is to learn how nanoparticle aggregation, joule heating, and a heat source affect the behavior of an ethylene glycol-based nanofluid as it flows over a permeable, heated, stretched vertical Riga plate and through a porous medium. Numerical solutions to the present mathematical model were obtained using Mathematica's Runge-Kutta (RK-IV) with shooting technique. In the stagnation point flow next to a permeable, heated, extending Riga plate, heat transfer processes and interrupted flow phenomena are defined and illustrated by diagrams in the proposed mixed convection, joule heating, and suction variables along a boundary surface. Data visualizations showed how different variables affected temperature and velocity distributions, skin friction coefficient, and the local Nusselt number. The rates of heat transmission and skin friction increased when the values of the suction parameters were raised. The temperature profile and the Nusselt number both rose because of the heat source setting. The increase in skin friction caused by changing the nanoparticle volume fraction from φ=0.0 to φ=0.01 for the without aggregation model was about 7.2% for the case of opposing flow area (λ=−1.0) and 7.5% for the case of aiding flow region (λ=1.0). With the aggregation model, the heat transfer rate decreases by approximately 3.6% for cases with opposing flow regions (λ=−1.0) and 3.7% for cases with assisting flow regions (λ=1.0), depending on the nanoparticle volume fraction and ranging from φ=0.0 to φ=0.01, respectively. Recent findings were validated by comparing them to previously published findings for the same setting. There was substantial agreement between the two sets finding

Heat transfer analysis in Darcy Forchheimer flow of hybrid nanofluid for multiple shape effects over a curved stretching surface

Nanoparticles and their shapes have practical implications in engineering and industrial processes. For this purpose, we will present a comprehensive study of nanoparticles shape effects on the flow over a curved stretch surface. The Runge Kutta (RK) method is used to numerically solve the non-linear differential equations resulting from the transformation of the flux process equations. First, the equations are transformed into a collection of first-order problems using the shooting method; next, the RK method is applied to solve them. Graphical analysis is carried out to study the implications in flow and heat transfer. Further, changes in skin friction coefficient and local heat transfer rate are graphically displayed and comprehensive discussions on the results are provided

Significance of entropy generation and nanoparticle aggregation on stagnation point flow of nanofluid over stretching sheet with inclined Lorentz force

It is well established that adding a certain number of nanoparticles to a nanofluid improves its thermal conductivity. The cause of this remarkable development is as of yet unidentified. Therefore, knowing the kinematics of nanoparticle aggregation is essential for determining the correct thermal impact of nanoscale particles. There are several potential technical and industrial uses for nanomaterials. From the perspective of these many application aspects, this paper examines the Al2O3-H2O nanofluid flow caused by a permeable stretching surface with the influence of an inclined Lorentz force and viscous dissipation. Entropy generation on nanofluid stagnation point flow with the influences of heat generation/absorption, nanoparticles aggregation with suction are also discussed in the current context. With and without nanoparticle aggregation, measurements of velocity, temperature, and entropy production are made. By applying the necessary transformations for heat and motion, ordinary differential equations may be derived from partial differential equations in certain circumstances. For the solution of ordinary differential equations, the Bvp4c technique is used. The effects of several dimensionless limitations on velocity, temperature, and entropy production, skin friction, and Nusselt number profiles are investigated, both when nanoparticle aggregation is present and when it is not. It is concluded that the velocity field is boosted for the velocity ratio and inclined Lorentz force parameters, while the temperature and entropy generation rise for the nanoparticle volume fraction, angle of inclination and Eckert number parameters. The rate of heat transmission improves as a result of the addition of ϕ and ε respectively. When the suction parameter of ε=0.5 is used for the aggregation model, it is claimed that there is an increase of roughly 13.2478% in the heat transfer rate. Temperature and entropy generation profiles for all pertinent parameters is higher for nanoparticles that have aggregated together as opposed to nanoparticles that have not aggregated together. This research was compared with previously published results to validate the findings, and great agreement was found

Impact of ciliated walls on peristaltic flow of Rabinowitsch fluid through flexible tube with heat/mass transfer

Current work focuses attention on discussing the peristaltic flow of Rabinowitsch fluid through ciliated walled inclined circular tube. The related mathematical equations are modelled in the presence of thermal radiation and connective conditions at the boundaries. In addition, mass transfer with Soret effects is also given consideration. Lubrication approach is adopted for simplification of the nonlinear set of equations. The resulting equations are then solved exactly by implementing various command of MATHEMATICA subject to relevant boundary conditions. Results are discussed for various flow quantities like velocity (u), temperature (θ), concentration (φ), pressure gradient (dp/dz), pressure rise (Δp) and heat transfer (Z). The effect of various parameters on velocity are discussed separately for shear thinning and shear thickening fluid in detail. Convective heat transfer is found to be greater at the boundaries resulting in decreased temperature there

Solidification acceleration of phase change material in a horizontal latent heat thermal energy storage system by using spiral fins

One important solution for societies to overcome the energy crisis is to manage the production and storage of energy. In this regard, the latent heat energy storage systems have recently gained a remarkable attention. Hence, this work is devoted to enhance performance of a latent heat tube-shell storage system by using novel spiral fins with different geometrical characteristics. Different number of vanes have been employed on the fins with constant twisting pitch. The different cases studied include the cases with single-fin, double-fin, triple-fin and quadruple-fin. These fins have been employed to augment heat transfer from a low temperature fluid to the PCM inside the shell container during the solidification process. To model the phase change, the enthalpy-porosity technique has been adopted. Various parameters have been used to assess the functionality of the system including the Nusselt number, total solidification time, temperature, solid fraction evolution, solid fraction contours, temperature contours and etc. The results demonstrated that the triple-fin and double-fin cases took the advantage in heat transfer augmentation and discharging process while the case with single-fin showed the worst performance. The results indicated that fin number is not the only influencing factor and fin length also has a dominant role