Constant Temperature Heat Source Research Articles

Expression of Concern: Numerical study of entropy generation in the convection heat transfer of nanofluid inside a tilted closed compartment with five constant-temperature heat sources in the presence of a magnetic field

Expression of Concern: Numerical study of entropy generation in the convection heat transfer of nanofluid inside a tilted closed compartment with five constant-temperature heat sources in the presence of a magnetic field

Simulation of heat transfer in a nanoparticle enhanced phase change material to design battery thermal management systems: A lattice Boltzmann method study

BackgroundIn the present paper, the natural convection of phase change material (PCM) in a T-shaped cavity is investigated by the lattice Boltzmann method (LBM). In recent years, LBM has become a powerful method for computational modeling of a broad variety of complex fluid flow concerns, including the simulation of the melting process in PCMs. MethodsThe enclosure contains CuO nanoparticles and two constant temperature heat sources on the sidewalls of the cavity. The obtained results are presented in different Rayleigh numbers (Ra=103–105), cavity angles (θ = 0 − 90), the volume fraction of nanoparticles (ϕ = 0 − 0.03), and aspect ratios. Results show that the PCM melting speed is lower for low Rayleigh numbers compared to when the number is equal to 105 by 50%. To alter melting time in a specific Ra number, adding nanoparticles, changing cavity slope, and aspect ratio are investigated. Results show that the melting rate is little affected by the addition of nanoparticles but, generally adding nanoparticles delays PCM melting. Significant findingsRaising the distance between the battery and the top of the cavity is known to delay melting by 70% when the distance ratio increase from H1/H = 0.25 to 075, whereas increasing or lowering the distance between the batteries does not affect the melting time. Such a study can be used to design battery thermal management systems (BTMS).

Latent heat storage composites composed of Al‐Si microencapsulated phase change material and alumina matrix

AbstractLatent heat storage (LHS) using phase change materials (PCMs) is expected for application to heat utilization at high‐temperature because it can provide a heat source of high density and constant temperature. Even among PCMs, metals/alloys are promising for high‐temperature operation. However, metals and alloys PCM are concerned about corrosion reactions with structural materials. As a result, micro‐encapsulated PCMs (MEPCMs) have been created in which the surface of alloy PCM is encased in a stable oxide coating. Since the surface of MEPCM is an oxide, the creation of structures with the function of LHS in a variety of shapes by combining MEPCM with sintering aids is also possible. In this study, composite PCMs were prepared by combining Al‐25 mass% Si MEPCM with fine α‐Al2O3 particles as a sintering aid. Consequently, PCM composites containing 70‐90 vol.% of the MEPCM and heat‐treated at 1200°C or 1300°C were successfully fabricated. In particular, a high heat storage density of 0.47 GJ m−3 was obtained under conditions containing 90 vol.% of the MEPCM and a heat treatment temperature of 1200°C, which is 1.6‐fold higher than that of existing sensible heat storage materials. Additionally, the composite PCM retained its shape and the latent heat capacity even after 300 cycles of cyclic testing. Thus, the high heat storage density and high durability of the composite PCM are expected to further promote the expansion of high‐temperature heat utilization in future studies.

MHD mixed convection of nanofluid in a cavity with isothermal local heater under an influence of velocity modulation of upper cold wall

Flow and heat transfer dynamics are extremely complicated phenomena in various enclosures. The mechanism of heat transmission and its enhancement due to the presence of nanoscale particles is an important research area. Owing to the broad significance of convection fluid flows with nanoparticles in different configurations and limitations, we have addressed in this study the mixed convection of aluminum oxide (Al2O3)-water nanosuspension in a closed chamber having a moveable upper boundary and a semicircular heat source of constant temperature. The simulation has been carried out with the impacts of thermal radiation and uniform horizontal magnetic field. Mass, momentum, and energy conservation laws have been used to develop the mathematical model for assessing the effectiveness regarding the thermal performance of nanoparticles. Objective of this analysis is to study the factors that improve the effectiveness of thermal transport in heat transfer machinery and heat recovery units used in engineering and industrial processes. Firstly, appropriate transformations are employed to alter the governing system into non-dimensional forms. The dimensionless system was then numerically solved for the wide range of emerging parameters. The system of governing equations has been solved on the basis of the finite difference method. Parabolic equations have been solved using the locally one-dimensional Samarskii scheme and then using the Thomas algorithm. An elliptic equation has been approximated by central differences and then solved by the successive over-relaxation method. The obtained distributions characterize the consequences of parameters on the fluid flow, establishment time of the quasi-stationary regime, and efficiency of heat transfer in the investigated cavity. Thus, it has been revealed that the effective energy removal can be obtained for ξ = 0.01 under an influence of magnetic field and nanoparticles addition.

Optimum energy efficiency in lunar in-situ water ice utilization

Lunar in-situ water ice utilization is considered an essential part of the future construction of Lunar Bases. However, the thermal conductivity of lunar regolith without water ice is extremely low, which seriously hinders the thermal mining of lunar water ice. In this study, we proposed a novel approach to optimize the energy efficiency of water ice thermal mining. In this method, a constant temperature heat source with a heating temperature selected according to the particle size of water ice was used to slow down the reduction rate of the thermal conductivity of icy soil. Our simulation results showed that the relatively high mining temperature led to the rapid sublimation of water ice near the heat source, reducing the thermal conductivity of the icy soil and the energy efficiency.A relatively low mining temperature decreases the sublimation speed of water ice and reduces energy efficiency. The particle size of water ice determined the decreasing rate of thermal conductivity of icy soil, thus affecting its optimum heating temperature. Using a constant temperature heat source at the optimal heating temperature, the energy efficiency of water ice mining could be increased by several orders of magnitude compared with constant power heating.

Numerical study of entropy generation in the convection heat transfer of nanofluid inside a tilted closed compartment with five constant-temperature heat sources in the presence of a magnetic field

Due to the significance of irreversibility, the entropy production of the natural and forced convection heat transfer of a nanofluid in a two-dimensional closed compartment is examined in the present paper. The side walls of the enclosed compartment are insulated, while the top and bottom walls of the compartment are cold and hot, respectively. The moving top wall causes forced convection in the closed compartment. Five rectangular heat sources are placed in the middle of the closed compartment to create free convection in the closed compartment. A horizontal magnetic field is applied to the closed compartment. The Bejan number (Be), the tilt angle of the closed compartment, the entropy generation, and the Richardson number (Ri) are all assessed as a function of the magnetic field's intensity. The Lattice Boltzmann method (LBM) is used for the numerical simulations. The results reveal that the entropy production is reduced with the closed compartment tilt angle up to 60°. Larger angles lead to an increment in entropy production. Angles of 0° and 60° are connected to the maximum and lowest amounts of entropy production, respectively. The Hartmann number (Ha) enhancement for strong forced convection (Ri = 0.1 and 1) intensifies entropy production. For weak forced convection (Ri = 10 and 100), an increment in the Ha reduces the amount of entropy production. Ri increase decreases the amount of entropy production.

Performance Analysis of Multistage Thermoelectric Cooler with Water-Cooled

A finite time thermodynamic model of the multistage thermoelectric cooler with constant temperature heat source and water-cooled is established, and the calculation method of input power and coefficient of performance (COP) are given. The influence of key parameters on device performance under different working conditions is given. The effects of the working current and cross-sectional area of the thermoelectric unit on refrigerating capacity, COP, thermodynamic perfection, module utilization coefficient, and coordination performance coefficient are analyzed. The results show that there are optimal working currents and optimal cross-sectional area, which can make the performance of the device reach the best. The evaluation index also provides a reference for realizing different goals and optimization.

Investigation of cascade high temperature heat pump optimal design theory based on experiment supporting multi-objective optimization

Cascade high temperature heat pump has been drawing wide attention due to its impressive application prospects to upgrade the low temperature heat. However, the relatively high initial cost has been hindering its application. Therefore, a refined design theory considering both thermodynamic performance and economic performance has been proposed. Herein, multi-objective optimization model with the improved non-dominated sorting genetic algorithm was adopted to determine the key design parameters: evaporation temperature and condensation temperature. The cost of per exergy output and initial cost of per heat capacity were cited as objective functions. Based on the Pareto fronts and the technique for order preference by similarity to an ideal solution method, the most favorable evaporation temperature and condensation temperature were determined. For the working condition of constant heat source temperature and different output water temperatures, the optimal evaporation and condensation temperatures were 15.27 °C and 101.93 °C, 13.06 °C and 111.77 °C, 11.44 °C and 121.58 °C, which denoted that the optimal condensation temperature increased along with the output water temperature but the differences between them were kept with a rather small range. For the working condition of constant output water temperature and different heat source temperatures, the optimal evaporation temperature and condensation temperature were 11.21 °C, 11.44 °C, 11.95 °C and 121.8 °C, 121.58 °C, 122.03 °C, respectively. Conclusion that optimal design evaporation temperature and condensation temperature were barely influenced by the heat source temperature could be drawn.

Thermodynamic Optimization of the Ethylene Oligomerization Chemical Process

The use of olefin oligomerization in the synthesis of liquid fuel has broad application prospects in military and civil fields. Here, based on finite time thermodynamics (FTT), an ethylene oligomerization chemical process (EOCP) model with a constant temperature heat source outside the heat exchanger and reactor pipes was established. The process was first optimized with the minimum specific entropy generation rate (SEGR) as the optimization objective, then multi-objective optimization was further performed by utilizing the algorithm with the minimization of the entropy generation rate (EGR) and the maximization of the C10H20 yield as the optimization objectives. The results showed that the point of the minimum EGR was the same as that of SEGR in the Pareto optimal frontier. The solution obtained using the Shannon entropy decision method had the lowest deviation index, the C10H20 yield was reduced by 49.46% compared with the point of reference and the EGR and SEGR were reduced by 59.01% and 18.88%, respectively.

Numerical Study on Natural Convection of Alumina-water Nanofluid in a Square Cavity with Two Localized Heat Sources on Adjacent Surfaces

In the present work, results of a numerical study carried out using finite volume method, to investigate the fluid flow and heat transfer characteristics of Alumina ( Al2O3 ) nanoparticles in the base fluid (water) in a square cavity under natural convection mode are presented. The Semi Implicit Method for Pressure Linked Equations (SIMPLE) algorithm was used to solve the discretized momentum and energy equations. Constant temperature heat sources of same strength are placed on bottom and left vertical surfaces whereas the right surface was kept cold, while the top surface was maintained as adiabatic. The impact of Rayleigh number (RaN) ( 1000 to 106 ) and nanoparticles volume fraction (Φ = 0 %, 5 %, 10 %, 15 % and 20 %) on fluid and heat flow characteristics were numerically investigated and presented in the form of streamlines, isothermal lines, mid line horizontal and vertical velocity components, local Nusselt number ( Nuloc ) and average Nusselt number ( Nuavg ). The obtained results indicate, for lower RaN ( i.e; 103 ), conduction dominates over convection near heated surfaces and results in lower fluid velocities and poor heat transfer. For higher values of RaN ( RaN = 105 and 106 ) and volume fraction of nanoparticles, there was a significant increase in mid horizontal and vertical velocity components, Nuloc and Nuavg due to increase in convective heat transfer and thermal conductivity of nanofluid.

Optimization and effect of wall conduction on natural convection in a cavity with constant temperature heat source: Using lattice Boltzmann method and neural network algorithm

In this paper, the natural convection (NC) of the Al2O3–H2O nanofluids (NFs) in a cavity with a heat source in its center is numerically investigated. A two-dimensional simulation of the cavity has been performed for this purpose. The D2Q9 LBM was used for the simulation. Then, the largest Nu was obtained using the artificial neural network (ANN). The ANN has been trained in such a way as to be able to automatically modify the input values and to estimate the largest heat transfer. With an increase in the Ra and nanoparticle volume fraction (NPVF), the heat transfer increases, but it decreases with an increase in the Ha number. In addition, the maximum heat transfer rate occurs for parallel magnetic field (MaF) and the heat transfer rate decreases with an increase in the MaF angle. As the size of the isothermal heat transfer and cold walls increase with a rise in the thermal conductivity coefficient, heat transfer also increases. With an increase in the thermal conductivity coefficient, no significant change was observed in the rate of heat transfer after an initial increase in the Nu. The largest Nu occurs at the largest Ra, largest MaF angle, and weakest MaF. The ANN predicted this value to be 7.0703, which is 2.3% different from the exact value resulting from the predicted computations.

Exergy Evaluation of the Measured Performance of Our Ejector Refrigeration Cycle

There is huge amount of unused heat resources being lower than 373 K not only in industries but also in increasing PV-solar-panels. Utilization of such thermal energy for cooling can greatly reduce the consumption of electricity especially in summer. We focus on the development of an ejector refrigeration-cycle system for enhancing such heat-utilization. We measured the performance of the testing device of an ejector refrigeration cycle. Confirmation of operation-stability of our testing device and exergy evaluation of the performance are reported here. Exergy evaluation is a useful tool for heat-pump systems because thermal energy can be treated as the energy being as the equivalent value of electricity. The ideal exergy-efficiency is always 100 %, which differs from COP being greater than unity. It is important that COP doesn’t show the ideal value. Calculations of the exergy assuming constant-temperature heat-sources just as treating of Carnot cycle in Thermodynamics from the principle, and the other considering the temperature change as used in heat-transfer processes for rational heat-exchanger designing are also reported here. As a result, both exergy efficiencies are still very low, which suggests us more improvements to be possible and to be required for the next stage of the development.

Nanoparticles and metal foams for heat transfer enhancement of phase change materials: A comparative experimental study

This paper presents an experimental work addressing the low thermal conductivity of phase change materials (PCMs), which is a major challenge associated with PCMs, when used as thermal energy storage solutions. This study experimentally compares the effectiveness of using copper oxide nanoparticles and copper metal foams, together and separately, for improving the performance of a selected PCM (RT44HC) during melting and solidification processes and for different heat transfer configurations. Four samples of pure PCM (as the base case), PCM-nanoparticle, PCM-metal foam, and PCM-nanoparticle-metal foam were tested. Two types of melting experiments were conducted: at constant temperature and with constant heat flux heat sources. The solidification experiments were conducted with a constant temperature heat sink. The results of the experiments showed that, compared to the base case of pure PCM, the heat transfer rate inside the PCM-nanoparticle, PCM-metal foam, and PCM-nanoparticle-metal foam composites increased by, respectively, about 13%, 17%, and 24% during melting with constant temperature heat source, about 24%, 26%, and 65% during solidification with constant temperature heat source, and about 7%, 11%, and 12% during melting with constant heat flux heat source. This study also shed light on some other challenges associated with using PCM as well as the role and the practicality of applying above-mentioned thermal techniques in addressing them.

Energy efficiency of membrane distillation: Simplified analysis, heat recovery, and the use of waste-heat

Membrane distillation (MD) is a thermal desalination process that is advantageous due to its ability to harness low-grade waste heat to separate highly saline feedstock. However, like any thermal desalination process, the energy efficiency depends on the ability to recover latent heat from condensation in the distillate. In direct contact MD (DCMD), this can be achieved by integrating a heat exchanger (HX) to recover latent heat stored in the distillate stream to preheat the incoming feed stream. Based on the principle of equal heat capacity flows, we derive a simple and intuitive expression for the optimal flow rate ratio between the feed and distillate streams to best recover this latent. Following the principle of energy balance, we derive simple expressions for the specific thermal energy consumption (SECth) and gained output ratio (GOR) of DCMD with and without a coupled HX for latent heat recovery, revealing an intuitive critical condition that indicates whether DCMD should or should not be coupled with HX. As MD is attractive for its ability to use low-grade waste heat as a heat source, we also evaluate the energy efficiency of DCMD powered by a waste heat stream. A waste heat stream differs fundamentally from a conventional constant-temperature heat source in that the temperature of the waste heat stream decreases as heat is extracted from it. We discuss the implication of this fundamental difference on energy efficiency and how we should analyze the energy efficiency of DCMD powered by waste heat streams. A new metric, namely specific yield, is proposed to quantify the performance of DCMD powered by waste heat stream. Our analysis suggests that, for a single-stage DCMD powered by a waste heat stream, whether implementing latent heat recovery or not only affects conventional metrics for energy efficiency (e.g. SECth and GOR) but not the specific yield. Overall, this analysis presents an intuitive and important framework for evaluating and optimizing energy efficiency in DCMD.

Numerical simulation of nanofluid convective heat transfer in an oblique cavity with conductive edges equipped with a constant temperature heat source: Entropy production analysis

In the present work, the entropy production of Al2O3-water nanofluid in an oblique cavity enclosure is examined. The side edges of the enclosure are at lower temperature and the top and down ones are insulated. Two thick conductive walls are placed on the low temperature edges. Furthermore, the center of the enclosure is equipped with a constant high temperature heat source. The enclosure is exposed to an inclined magnetic field. The governing nonlinear partial differential equations are continuity, Navier–Stokes and energy equations. These equations are solved using an open-source CFD software package (OpenFOAM). The influence of effective parameters includes Ra number, Hartman number, the angles of magnetic field and enclosure, nanoadditives concentration, and aspect ratio on the entropy production and the Bejan (Be) number are investigated. The results show that the minimum entropy production happened at low power magnetic fields. With the increase of the slope of the cavity, the entropy production rises. The addition of nanoadditives leads to an intensification in the entropy production and a reduction in the Be number.

Using finite volume method for simulating the natural convective heat transfer of nano-fluid flow inside an inclined enclosure with conductive walls in the presence of a constant temperature heat source

In the present work, natural convective heat transfer of water/Al2O3 nano-fluid in an inclined square enclosure is investigated. The side walls of the cavity are cold and the upper and lower ones are insulated. A wall with a thermal-conductivity of 100 and a thickness of 0.5 is located on the cold walls. Moreover, there is a constant temperature heat source in the center of the enclosure. The enclosure is located under the influence of an inclined magnetic field (MF). The governing equations were solved using the finite volume method (FVM) and solved using the SIMPLE algorithm. The results show that the heat transfer rate intensifies up to 3.11 times with intensifying the Rayleigh number (Ra). The maximum heat transfer occurred at weak magnetic fields. By augmenting the angle of the enclosure, the heat transfer rate on the right and left walls intensifies by 33% and declines by 55%, respectively. The heat transfer rate on the right wall intensifies by 14% by augmenting the angle of the MF. The addition of nano-additives also results in intensification in the heat transfer rate.

Mixed convection and radiation studies on thermally developing laminar flow in a horizontal square channel with variable side heated wall

In this work presents the interaction of mixed convection and surface radiation heat transfer studies on thermally developing laminar airflow in a differentially heated horizontal channel with isothermal vertical walls and adiabatic top and bottom walls. The side heated isothermal wall incorporated a heater, and the opposite wall acts as a constant temperature heat source. The major thermal and geometric parameter includes variable wall temperatures, wall surface emissivity, Reynolds number, heat flux and air chosen as the working medium. Due to the wall surface emissivity of the channel, the internal wall surface radiation will significantly change the rate of total heat transfer inside the channel. Therefore, the current research focuses on the mixed convective and surface radiation heat transfer process happening in laminar flow over a horizontal channel. Radiative heat transfer is determined with the help of experimental results and the use of the numerical scheme. The results presented here show the rate of convection heat transfer and the distribution of surface temperature along the walls inside the channel. The flow configuration the channel inside is pictured by passing smoke at the bottom of the entrance section. The quantitative and qualitative results showed that the emissivity of walls and other thermal and geometric parameters significantly affected the overall heat transfer improvement from the square channel.

Heat-Transfer Enhancement Incorporating Fin-Like Structures Inside Droplet on Hydrophobic Surface

Enhancement of droplet heat transfer on a hydrophobic surface is examined via introducing the fin-like structures inside the droplet without altering the wetting state of the surface. A solution crystallization of polycarbonate surface is carried out and the functionalized silica particles are deposited onto the crystallized surface to create the hydrophobic surface characteristics. The ferrous particles (Fe2O3) are locally spread onto the hydrophobic surface and, later, manipulated by an external magneto-static force generating various configurations of fin-like structures inside the droplet. The droplet with fin-like structures is heated from the hydrophobic surface through introducing a constant temperature heat source. Flow and temperature fields inside the droplet are simulated in line with the experimental conditions. It is found that changing the configuration of the fin-like structures in the droplet modifies significantly the flow and temperature fields inside the droplet. The Bond number remains less than unity for all configurations of the fin-like structures while demonstrating the importance of the Marangoni current over the buoyancy current in the flow field. The presence of the fin-like structures lowers the difference between the fluid bulk and the minimum temperatures inside the droplet and improves considerably the heat transfer rates and the Nusselt number.

Investigation of Performance Enhancements for Air Brayton/ORC Combined Cycles for Small (~ 2 MWe) Power Systems and a Moderate Heat Source Temperature

Attractive power generation cycles from economic and sustainability considerations are highly efficient and require minimal cooling water. Air Brayton cycles (ABCs) require no cooling water; however, air Brayton combined cycles (ABCCs), which require a minimal water, are more efficient. Several ABCs and ABCCs designed to operate at 5 MWth (~ 2 MWe) were evaluated in temperate environments with a constant temperature heat source at 650°C. An Organic Rankine cycle (ORC) operating with r143a was selected for the bottoming cycle based on previous analyses. Efficiency can increase from 22.0% in the standard ABC to 39.8% in the ABCC. Various performance enhancements were explored, including air inlet cooling, ORC regeneration, and ORC reheat. For the recuperated ABCC with evaporative cooling, the capital cost is < US$17M and the payback period is < 13 years. The major exergy losses are 56.4% spent as work at the ABC turbine and 23.5% lost at the heat source.

Numerical study of boiling around wires and influence of active or passive neighbours on vapour film dynamics

In the present study pool boiling around a heated wire has been simulated at atmospheric pressure, considering the wire to be at the constant temperature heat source. Film dynamics, bubble nucleation, growth, merging and pinch off has been observed in with perfectly smooth and uniform horizontal wire without any nucleation site. Effect of increase of the degree of superheat on boiling rate, measured in terms of vapour generation and bubble population has been studied next. Similar studies have been done by inclining the wire at different angles (10°, 45°, 60° and vertical), where axial and azimuthal bubble sliding and more efficient bubble merging have been observed. Bubble size and population distribution have been investigated along the length of the inclined wire to link up with corresponding enhancement in boiling rate. For understanding the effect of active and passive neighbour, horizontal boiling around two-wire system has been studied. First horizontal wires have been stacked at the same vertical plane and different active and passive combination have been simulated. Also boiling from two active wires placed at same horizontal plane has been studied for different wire spacings. For all these cases bubble life cycle has been investigated to understand the role of neighbouring wire. Effect of these neighbouring wires on boiling heat transfer and vapour generation rate has been reported.