As the Maxwell-type boundary treatment can automatically capture temperature jump on boundary, it is widely used in gas flow simulation like Lattice Boltzmann method and Direct simulation Monte Carlo method. In present study, diffuse-specular Maxwell-type boundary with a diffusive fraction (a), which decides the mechanism of interaction between gas molecules and boundary, is realized in UGKS simulation. This diffuse-specular boundary can recover diffuse Maxwell boundary when a = 1.0, which proves the reliability of present boundary treatment. The influence of diffusive fraction on wall temperature jump under Knudsen number ranging from 0.001 to 1.0 is tested. The test cases are steady and unsteady state conditions of heat conduction and Couette flow between two infinite plates setting at specified temperatures. It is found that: 1) for cases of Knudsen number ranging from 0.01 to 1.0, owing to the loss of influence from equilibrium part on evolution of boundary gas distribution, the relative temperature jump increases when diffusive fraction varies from 1.0 to 0.25, this phenomenon is especially obvious on extreme point part in unsteady state cases; 2) for cases of Knudsen number equaling to 0.001, diffusive fraction has no significance influence on temperature jump as the temperature jump is less noticeable for such condition. Present study will help the further researches of heat transfer in rarefied gas.

In this study, a numerical analysis of turbulent flow heat and mass transfer in the cross-flow transport membrane condenser (TMC) based heat exchange was carried out. The heat exchanger under investigation was designed to recover both sensible and latent heat due to transport of heat and mass through a nanoporous ceramic membrane in the bundle of tubes of the heat exchanger. The shear stress transport SST k-ω turbulence model was used to model the turbulent flow of the flue gas mixture. The condensation rate of the water vapor from the flue gas were calculated using a mixed condensation model. The mixed model was based on the capillary condensation and wall condensation in the membrane tube. The numerical study was focused on the investigation of the impact of the turbulence intensity of the flue gas at various inlet conditions, such as Reynolds numbers and temperatures, on the heat and mass transfer and pressure drop characteristics. The numerical results were validated against the experimental results reported in the literature. Different tube diameters were used in the simulation, with the Reynolds number varied from 3000 to 10000. The results showed that an increase in turbulence intensity led to a significant increase in the turbulent kinetic energy, condensation rate, average convective Nusselt number and change on the pressure drop in the heat exchanger. The effects of inlet flow Reynolds number and tube diameter on the heat and mass transfer were also presented and discussed.

Electromagnetic (EM) heat exchangers (HX) are critical components in power beaming applications where EM waves are radiated towards an EM HX, which then converts incident energy into heat or mechanical work. An EM HX consists of a lossy ceramic undergoing EM heating, and a fluid flow maintaining thermal contact transfers heat from the ceramic. Temperatures during high-power EM processing of ceramics materials such as zirconia suggest that liquids would be in gaseous phase, so models of EM HX with compressible gas dynamics may provide insights into experimental scenario. As a first step, we consider an EM HX such that plane Poiseuille flow of an incompressible coolant whose density drops linearly with temperature is situated above a lossy ceramic material. Compressible effects are negligible, but density gradients within the fluid can give rise to buoyancy-driven flow under the action of gravity, which may affect the performance of the device. We determine the power of incident waves at which Bénard convection is initiated, through a linear stability analysis, in the fluid layer. We show that in case of temperature dependent ceramic loss factor, perturbations in temperature give rise to an electric (fringe) field which then feeds back into the system promoting the Bénard instability.

Heat transfer over rotating surfaces is of particular interest in rotating machinery such as gas turbine engines. The rotation of the gas turbine disc creates a radially outward flow on the disc surface, which may lead to ingress of hot gases into the narrow cavity between the disc and the stator. Impingement of cooling jet is an effective way of cooling the disc and countering the ingress of the hot gases. Present study focusses on investigating the effect of introducing pin-fins over the rotating disc on the heat transfer. The jet Reynolds number has been varied from 5000 to 18000, and the rotating Reynolds number has been varied from 5487 to 12803 for an aluminum disc of thickness 6.35mm and diameter 10.16 cm, over which square pins have been arranged in an inline fashion. Steady state temperature measurements have been taken using thermocouples embedded in the disc close to the target surface, and area average Nusselt number has been calculated. The effects of varying the height of the pin-fins, distance between nozzle and the disc surface and the inclination of the impinging jet with the axis of rotation have also been studied. The results have been compared with those for a smooth aluminum disc of equal dimensions and without any pin-fins. The average Nusselt number is significantly enhanced by the presence of pin fins. In the impingement dominant regime, where the effect of disc rotation is minimal for a smooth disc, the heat transfer increases with rotational speed in case of pin fins. The effect of inclination angle of the impinging jet is insignificant in the range explored in this paper (0° to 20°).

Transient numerical simulations have been carried out to mimic and analyse the transmission of various species resulting from human sneezing. The extent of the spread of sneezed air and associated droplets is also investigated based on various parameters. A 2D geometry of the human face is considered that captures the true topology and the outlet characteristics of the exhaled air mixture. Numerous parameters are required to be considered to capture the out-coming mixture trajectory and to track its concentration evolution as it enters and entrains with the surrounding air. These parameters include the velocity of the exhaled air mixture, the extent of mouth opening, the distribution of the mixture fraction, and its mist content. A multi-species Eulerian flow with discrete phase Lagrangian particles is considered. The results include the spatial and temporal distributions of the species and their velocity contour plots. Specifically, the concentration of the exhaled species is captured both spatially and temporally at several hypothetical stations within the computational domain, and away from the source to substantiate/refute the current recommended social distance parameter.

The influence of oscillation and heat transfer on the lift and drag coefficients over a circular cylinder is numerically studied in this work. Temperature difference of 300, 600 and 900 K is used between the cylinder wall and the incoming fluid flow for Reynolds number of 100. Air is used as the fluid and the temperature dependent properties of air are used for the analysis as a significant change in the properties of air incurred. Numerical simulation is done on Ansys/fluent with O-type mesh and the vibration in the circular cylinder is induced using user defined function. The vibration of the cylinder in streamwise direction is induced at a frequency ratio of 0.5, 1, and 2, with the natural frequency of the cylinder being 2.5 Hz marking its Strouhal number. It is observed that for all the induced frequencies, the forcing function interacts with the natural frequency of the system, and the beating phenomenon spectrum is observed, where two distinct frequencies appear which correspond to the sum and difference between the natural and the forcing frequency. At the frequency ratio of 0.5 (1.25 Hz), the spectrum of lift coefficient is characterized with three peaks centered at 2.5 Hz (natural frequency), 3.75 Hz (sum) and 1.25 Hz (difference). Oscillating the isothermal cylinder at a frequency ratio of 0.5 caused a negligible increase in the rms value of the lift coefficient by 2.13%, drag coefficient by 0.17%, and had no effect on the natural frequency of the system, however at a frequency ratio of 2, a drastic increase in the rms value of lift coefficient by 137.4% and drag coefficient by 13.9% occurred, indicating the lock-on regime. As compared to the stationary isothermal cylinder, heating the cylinder 300K above the incoming flow, decreased the rms value of the lift coefficient by 62.7% and the natural frequency by 16%, while increased the drag coefficient by 7.3%. The results show that heating of cylinder in cross-flow is equivalent to running the flow at a reduced Reynolds number and in the laminar region, this is associated with lower Strouhal number and lower amplitude of lift but a higher drag.

Microchannel heat sinks have attracted much attention due to its superiority of removing high heat flux with a very small size, which recently have been applied in the electronic cooling. The flow boiling of deionized water in the ultrahigh-aspect-ratio copper microchannels was experimentally investigated in this work. The heat sink consists of 20 rectangular straight microchannels (5000μm × 200μm) with the hydraulic diameter of 385 μm and the aspect ratio of 25. The experiment was conducted with the heat flux range of 39.7–1368 kW/m 2 and the mass flow range of 21.3–41.6 kg/(m 2 ·s) (the inlet temperature of 30°C). The flow patterns in the ultrahigh-aspect-ratio microchannels were captured using the visualization technique and four flow regimes (bubbly flow, slug flow, churn flow and annual flow) were clearly observed. The nucleate boiling sites increase significantly in the depth direction on the large-area side wall at bubbly flow. An obvious delay for flow patterns transformation in the upper regions compared with the lower region of microchannel was found. The bubble sliding was observed with a large sliding distance compared to the conventional low-aspect-ratio microchannels, which leads to an enhanced the heat transfer. It shows that the higher effective heat flux and the lower pressure drop were achieved in ultrahighaspect-ratio microchannel due to its special flow patterns in the channel and large surface area in the limited volume.

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.

Solar photovoltaic (PV) system harness the energy from the sunlight and convert into clean electricity to power homes and businesses. During an operation the solar panels get hot, the electrons inside the solar cells pick up that extra heat energy which puts them in a more excited state and when they are already excited, they have less room to absorb the energy from sunlight. As a result, electrical performance of a PV system reduces with increase in solar cell temperature. Efficiency of PV panels can be retained by establishing a hybrid PV-Thermal (PV-T) system. In this study, container filled with phase change material (PCM) embedded in porous metal is attached to back surface of the PV cell. As well as, to extract the excess heat from the PV cell, water is used as a heat transfer fluid (HTF) with constant mass flow rate of 30 kg/hr. During the simulation melting rate of PCM, amount of latent heat energy stored, thermal, electrical and overall efficiencies of the PV panel is studied and compared with the conventional PV-T system. The results show the enhanced melting fraction of PCM by 6% and 8% for the PV-T/PCM/Cu and PV-T/PCM/Al system, respectively compared with PV-T/PCM system. Moreover, in comparison with the conventional PV-T system, the overall efficiency of the PV-T/PCM/Cu and PV-T/PCM/Al is increased by 10.62% and 8.80%, respectively.

Heated and unheated flows with forced convection over two fixed circular cylinders in tandem are studied numerically for 80 ≤ Re ≤ 250 and 1 ≤ T* ≤ 2.3. Three different spacing ratios (L/D) = [2, 4, 8] are considered under three heating conditions. The scenarios considered are (1) heated upstream and unheated downstream cylinders, (2) unheated upstream and heated downstream cylinders and (3) heated upstream and downstream cylinders. These scenarios represent the limiting case for a cross-flow heat exchanger, where the downstream tubes are at increasingly lower or higher temperature for cooling or heating, respectively. The global aerodynamic forces on the cylinder as vortices shed was investigated. The flow is visualized by plotting the streamlines, temperature fields, and velocity magnitude contours for the different spacing ratios and compared to the flow regimes in literature namely, Extended-body, Reattachment, and Co-shedding regimes. The drag and surface heat transfer coefficients are analyzed for different scenarios. The effect of heating on the fluid properties and the resulted wakes in the flow are found to be strongly influenced by Re and L/D. The scenario of heated upstream and unheated downstream cylinders was found to increase the mean drag coefficient C d on the upstream cylinder for L/D = 2 & 4 but is not as evident for the downstream cylinder. The heat transfer coefficient h on the upstream cylinder remained approximately the same regardless of a heated or unheated downstream cylinder. In contrast, h of the downstream cylinder decreases for the scenario of heated upstream and downstream cylinder.

In this study, we use infrared thermography to calculate local heat transfer coefficients of top and bottom heated flows of near-critical carbon dioxide in an array of parallel microchannels. These data are used to evaluate the relative importance of buoyancy for different flow arrangements. A Joule heated thin wall made of Inconel 718 applies a uniform heat flux either above or below the horizontal flow. A Torlon PAI test section consists of three parallel microchannels with a hydraulic diameter of 923 μm. The reduced inlet temperature (T R = 1.006) and reduced pressure (P R = 1.03) are held constant. For each heater orientation, the mass flux (520 kgm −2 s −2 ≤ G ≤ 800 kgm −2 s −2 ) and heat flux (4.7 Wcm −2 ≤ q″ ≤ 11.1 Wcm −2 ) are varied. A 2D resistance network analysis method calculates the bulk temperatures and heat transfer coefficients. In this analysis, we divide the test section into approximately 250 segments along the stream-wise direction. We then calculate the bulk temperatures using the enthalpy from the upstream segment, the heat flux in a segment, and the pressure. To isolate the effect of buoyancy, we screen the data to omit conditions where flow acceleration may be important or where relaminarization may occur. In the developed region of the channel, there was a 10 to 15 percent reduction of the local heat transfer coefficients for the upward heating mode compared to downward heating with the same mass and heat fluxes. Thus buoyancy effects should be considered when developing correlations for these types of flow.

In this work, we study the heat transfer performance and particle dynamics of a highly mass loaded, compressible, particle-laden flow in a horizontally-oriented pipe using an Eulerian-Eulerian (two-fluid) computational model. An attendant experimental configuration [1] provides the basis for the study. Specifically, a 17 bar co-flow of nitrogen gas and copper powder are modeled with inlet Reynolds numbers of 3 × 10 4 , 4.5 × 10 4 , and 6 × 10 4 and mass loadings of 0, 0.5, and 1.0. Eight binned particle sizes were modeled to represent the known powder properties. Significant settling of all particle groups are observed leading to asymmetric temperature distributions. Wall and core flow temperature distributions are observed to agree well with measurements. In high Reynolds number cases, the predictions of the multiphase computational model were satisfactorily aligned with the experimental results. Low Reynolds number model predictions were not as consistent with the experimental measurements.

This paper was removed from publication at the author’s request. October 2, 2020. Copyright © 2020 by ASME

Water temperature is an important parameter that influences the life near watercourses. The physical and chemical properties as the density and viscosity, gas solubility in water and in particular of the oxygen, the kinematics of the micro-biological and chemical phenomena affects not only the aquatic life but also the human life and animals living near the watercourse. In the Jiu river is discharged the water coming from the cooling towers of three thermo-electric power plants placed upstream. According to the recent researches realized by the EU Environmental Commission in Romania until 2050 the average temperature could rise by about 2°C. This paper presents the ability of a “neural networks” model to predict the water temperature in the lower part of the Jiu River. A database realized in over nine years in the measuring station Izbiceni, from the Olt County is used, concerning the water temperature (T w ) and the air temperature (T a ). The developed neural network is structured on five input variables and 9 hidden layers, 7 as sub-input and 2 as output. The stochastic time series is based on the separation of values for water T w and air T a in seasonal and daily fluctuations components. The schematic structure of the implemented neural network is presented, for obtaining the short-term and the seasonal components of the temperatures. The contact points are adjusted by the method of reprogramming after a gradient algorithm, in order to minimize the quadratic error between the output results and the relevant observed values. The performance of the neural network is based on estimation the thermal changes of the operation mode. For testing the numerical model, the calculations were performed using only the data recorded in nine years, in order to estimate a forecast for the next two years. The results are very close to the experimental recorded data and the obtained errors are less than 0.5°C. At the end of the paper are presented some results, conclusions and references.

Computational fluid dynamic simulations are conducted to compare the performance of three proposed VMD module designs. Key parameters, including concentration polarization coefficient ( CPC ), temperature polarization coefficient ( TPC ), mass flux, and module pressure drop, are evaluated to analyze flux performance and membrane wetting implications. The CFD simulations are conducted on a three-dimensional domain for each design with representative membrane properties. The Reynolds numbers evaluated are 500 and 1500. Contour plots are provided to gain insight into the flow-field characteristics. Stream-wise profiles and average values are provided as a means to compare the design performance. The results indicate that adding either net-type spacers or membrane corrugation to the feed channel provides performance enhancement over an empty channel design. It is found that both the net-type spacer concept and the corrugated membrane offer better flux performance over the baseline design (49%–60% for the net-type spacers and 62%–67% flux enhancement for the corrugated membrane), reduced concentration polarization coefficient (51% for the Re = 500 case and 45% for the Re = 1500 case for spacers and 58% for the Re = 500 case and 49% for the Re = 1500 case for corrugation), increased TPC (2%–3% higher), and increased wall shear stress over the baseline design. The most significant finding is that the corrugated membrane design accomplishes the listed performance improvement while yielding five times less pressure drop increase than the net-type spacer design.

The static and dynamic stability of Rayleigh-Bénard convection in a rectangular flow domain is computationally investigated. Sinusoidal vertical oscillations are applied to the system to provide dynamic flow stabilization. Stability maps are produced for a range of flow and heating conditions, and are compared to experimental measurements and linear stability analysis predictions from existing literature. Density variation is introduced through: 1) the Boussinesq approximation, 2) a linearly varying temperature dependent equation of state (EOS) and 3) the perfect gas EOS. Significant effects of choice of EOS on dynamic stability are observed. These weakly compressible flows are solved efficiently using an implicit numerical method that has been developed to solve the momentum, continuity, enthalpy and state equations simultaneously in fully coupled fashion. This block coupled system of equations is linearized with Newton’s method, and quadratic convergence is achieved. The details of these numerics are presented.

High surface charge density makes it possible for charge inversion to occur in the electrical double layer, in which cases, flow reversal appears for electroosmotic driven flow that is one of the best-known methods of fluid transport based on the electro-kinetic phenomenon. Reverse electroosmotic flow of an aqueous solution of KCl was studied using MD method in this work. Nonequilibrium molecular dynamics simulations were carried out to investigate the effect of temperature on velocity profiles of ions and water molecules confined in charged carbon nanotubes (CNTs) with a diameter of 3.0 nm. The results demonstrate that the temperature dependence of velocity of ions and water molecules is obviously different for various distances to the tube wall. It is found that the temperature has a direct effect on the ion velocity profiles, that is, as the temperature increases, the velocity of either counter-ion or co-ion in the diffuse layer increases significantly though the velocity drops sharply in the vicinity of the tube wall surface. As for water molecules, the MD simulations show plenty of velocity profiles, in which the velocity is slightly positive near the tube wall, but is negative in most of the rest region. The temperature effect on reverse electroosmotic flow is clearly observed, which is reflected in two aspects: the viscosity of the fluid and the velocity profiles of ion and water molecule.

Comfort criteria in building are increasing continuously. Multiple studies tend to use CFD approaches in order to predict and optimize the flow conditions accurately. One of the most relevant problematic is the extraction of the CO 2 by kitchen hoods. In the present work, a new solver, based on a buoyancy Boussinesq solver and a turbulent passive scalar transport equation, is developed in OpenFoam to model the heated CO 2 extraction by a rotating fan using the Multiple reference frame (MRF) approach. The new solver is first validated successfully against experimental results of a heated cavity and CO 2 diffusion. An improvement of 30% in the capture efficiency is reached by increasing the extraction flow rate of the fan from 100 to 300 cfm. An extraction efficiency of 100% is observed for flow rates above 330 cfm. The novelty of this study is to present a new open-source solver able to model the rotation of the whole fan, CO 2 transport and heat exchange simultaneously in steady state.

An experimental investigation of shell-side flow condensation heat transfer was performed on advanced three-dimensional surface-enhanced tubes, including a herringbone micro-fin tube and a newly-developed 1-EHT tube. An equivalent plain tube was also tested for performance comparison. All of the test tubes have similar geometry parameters (inner diameter 11.43mm, outer diameter 12.7mm). Tests were conducted using R410A as the working fluid at a condensation saturation temperature of 45 °C, covering the mass flux range of 10–55 kg/(m 2 ·s) with an inlet quality of 0.8 and an outlet quality of 0.1. Experimental results showed that the plain tube exhibits a better condensation heat transfer performance when compared to the enhanced tubes. Moreover, the mass flux has a significant influence on the heat transfer coefficient for shell-side condensation: the condensation heat transfer coefficient of plain tube decreases when the refrigerant mass flux becomes larger, while the heat transfer coefficient of herringbone tube shows a non-monotonic trend and the heat transfer coefficient of the 1-EHT tube gets higher with increasing refrigerant mass flux. Besides, A new prediction model based on the Cavallini’s equation was developed to predict the condensing coefficient of the three test tubes, and the mean absolute error of the improved equations is less than 4%.

The performance on the air side flow is often limited due to its lower heat transfer coefficient. This work is related to numerical simulation to study the significance of employing delta winglets in flat finned and wavy finned-tube heat exchangers. For this purpose, three-dimensional simulation data and a multi-objective genetic algorithm are employed. The angle of attack (α) of delta winglets and Reynolds number varied from 15° to 75° and 500 to 1300, respectively. Employing delta winglets has increased the heat transfer per unit temperature and per unit volume ( Z ) and the fan power per unit core volume ( E ) for both flat finned and wavy finned-tube heat exchangers. To achieve a maximum heat transfer enhancement and a minimum friction factor, the optimal values of these parameters ( Re and α) are calculated using the Pareto optimal strategy. For this purpose, CFD data, a surrogate model (neural network) and a multi-objective optimization genetic algorithm are combined. Results show that the performance of wavy finned-tube heat exchangers is higher than flat-finned tube heat exchangers which signify the importance of delta winglets in the wavy finned-tube heat exchangers.