Various Modes Of Heat Transfer Research Articles

Development of a one-dimensional model of a closed thermosiphon for cooling a spent-fuel pool

Using closed thermosiphons for cooling the spent-fuel pools of nuclear power plants requires calculation tools for simulating the thermal performance of long thermosiphons. The ability of the computer program THERESA to simulate the thermal performance of a vertical thermosiphon with water as the working fluid has been investigated. THERESA was used for simulating a cylindrical thermosiphon with a one-meter-long evaporator and a 32-mm inside diameter. The condenser section had the same inside diameter and a length of 0.5 m. Modelling the long thermosiphons of the type required to span the distance from the spent-fuel pool to the atmosphere requires that the effects of hydrostatic pressure in the evaporator and the pressure drop in the vapor between the evaporator and condenser sections be correctly simulated.The input data to THERESA are the outside wall temperatures of the evaporator and condenser. The thermal resistances between these two surfaces consist of conduction through the walls, the thermal resistance due to vapor-pressure drop in the adiabatic section between the evaporator and the condenser, and the various modes of heat transfer in the liquid phase of the working fluid. The heat transfer in the liquid of the condenser was simulated by a mixture of drop-wise and film-wise condensation. The heat transfer in the evaporator was simulated by a mixture of natural convection, nucleate boiling, and film boiling. After simulating the thermal resistances, THERESA solves for the heat-transfer rate of the thermosiphon.The simulation results were compared with data from experiments with steady-state boundary conditions. The trends in the experimentally derived heat-transfer coefficients as a function of heat-transfer rate were used to identify the type of phenomena that dominate the processes in the evaporator and the condenser. Additional insight into the instabilities occurring during these experiments led to new methods for calculating the heat-transfer coefficients in THERESA. The number of various heat-transfer phenomena simulated in THERESA was reduced to those with the trends that matched the trends in the experimental data. The heat-transfer coefficients for those dominant phenomena were modified in order to represent a time-averaged value over an entire instability cycle. Empirical constants for the modifications were obtained from the experimentally derived values.

Thermal performance analysis of a Solar parabolic dish concentrator system with modified cavity receiver

PurposeThis paper aims to present the numerical models and experimental outcomes pertain to the performance of the parabolic dish concentrator system with a modified cavity-type receiver (hemispherical-shaped).Design/methodology/approachThe numerical models were evolved based on two types of boundary conditions; isothermal receiver surface and non-isothermal receiver surface. For validation of the numerical models with experimental results, three statistical terms were used: mean of absolute deviation, R2 and root mean square error.FindingsThe thermal efficiency of the receiver values obtained using the numerical model with a non-isothermal receiver surface found agreeing well with experimental results. The numerical model with non-isothermal surface boundary condition exhibited more accurate results as compared to that with isothermal surface boundary condition. The receiver heat loss analysis based on the experimental outcomes is also carried out to estimate the contributions of various modes of heat transfer. The losses by radiation, convection and conduction contribute about 27.47%, 70.89% and 1.83%, in the total receiver loss, respectively.Practical implicationsAn empirical correlation based on experimental data is also presented to anticipate the effect of studied parameters on the receiver collection efficiency. The anticipations may help to adopt the technology for practical use.Social implicationsThe developed models would help to design and anticipating the performance of the dish concentrator system with a modified cavity receiver that may be used for applications e.g. power generation, water heating, air-conditioning, solar cooking, solar drying, energy storage, etc.Originality/valueThe originality of this manuscript comprising presenting a differential-mathematical analysis/modeling of hemispherical shaped modified cavity receiver with non-uniform surface temperature boundary condition. It can estimate the variation of temperature of heat transfer fluid (water) along with the receiver height, by taking into account the receiver cavity losses by means of radiation and convection modes. The model also considers the radiative heat exchange among the internal ring-surface elements of the cavity.

Buoyancy-driven melting and solidification heat transfer analysis in encapsulated phase change materials

Controlled melting and solidification in encapsulated phase change materials (PCMs) is of practical interest, for example, in latent heat storage applications. The choice of PCMs and the dynamic heat transfer characteristics during phase change - affecting the transient charging/discharging rates - are decisive for the energy and power density of the heat storage option. For example, highly conductive, high melting point (above 700 K) metal alloys have a potential advantage as a high energy and power density latent heat storage, compared to the widely used low conducting molten salt and paraffin wax. This advantage depends on the relative dominance of heat transfer modes that vary depending on the thermal properties of the PCM, shape of the encapsulation, and the load conditions, and must be quantified to warrant a fair comparison. We developed a 2D transient phase change model of encapsulated PCMs, accounting for phase change over a temperature range, volumetric expansion and contraction, and multi-mode heat transfer within the encapsulated PCM. The enthalpy-porosity method was used to model the phase change in a fixed grid. Validation was performed with literature data of low and high-conductivity PCM experiments (RT27 and lead). Two types of PCMs were subsequently investigated in detail: a high-conducting binary-eutectic alloy Al-12.6Si and a low-conducting commercially available RT27 paraffin wax (Rubitherm GmbH). The model was used to compare the phase change process and the strength of the various modes of heat transfer in the PCM filled cylindrical stainless-steel encapsulations in horizontal and vertical orientation with constant temperature walls. A full parameter study and non-dimensional analysis are presented for different PCMs. The results quantified the influence of boundary conditions, thermophysical properties and geometrical parameters on the phase change process and the contribution of the natural convection within the encapsulation. The non-dimensional analysis linked the melt fraction and heat transfer rates to a combination of the Fourier, Stefan, Rayleigh and Nusselt numbers. Fitting of the exponents of non-dimensional number groups to the heat transfer calculations allowed to provide general correlations for melting time, melt fraction, heat transfer rates, and characteristics of melting. Such correlations provide general understanding of the transient heat transfer in phase change media and provide engineering tools for designing, for example, a latent heat storage unit.

An experimental and a numerical analysis of the dynamic behavior of PCM-27 included inside a vertical enclosure: Application in space heating purposes

The paper deals with the study of the heat transfer phenomena within a vertical enclosure containing PCM integrated inside a test cell with the dimension of (0.5, 0.5, 0.5 m3). An experimental framework was conceived and installed in the Thermal Processes Laboratory in Borj Cedria, Tunisia. The experimental investigation was carried out by measuring the temperature and the heat fluxes exchanged throughout the PCM vertical enclosure. It was found that during the heating phase the temperature inside the test cell equipped with the PCM vertical enclosure is about 28 °C. It was also found that once the PCM vertical enclosure is omitted the cell temperature inside the test cell ranges between 29 and 40 °C. A complete model was also formulated by taking into account various modes of heat transfer inside the PCM vertical enclosure. The numerical model was performed by following the temperature changes at the bottom position and at the high position of the PCM vertical enclosure. The numerical model followed also the evolution of the melting front during the melting process. The model was then exploited for the evaluation of the melting front evolution during the charging and the discharging processes. Another TRNSYS simulation was achieved to evaluate the feasibility of integrating the PCM vertical enclosure in a real building envelop. The results of the TRNSYS simulation showed that the integration of a well dimensioned PCM vertical enclosure enhances the thermal comfort of the occupant by reducing the thermal fluctuation and by improving the thermal inertia of the walls.

Transient analysis of a circular foil gage in a convective and radiative environment

An analysis of a circular thin-foil gage is presented that includes transient effects, convective heat transfer, and an arbitrary time-varying boundary condition at the foil edge to account for fluctuations in cooling water temperature. The governing energy equation is solved using Laplace transform to obtain the temporal and spatial temperature distributions of the foil. Under constant temperature at the foil edge and constant thermal radiative heat flux, closed-form response curves for the gage under various modes of heat transfer are provided. Steady-state results are also presented as limiting cases.

Numerical simulation of fluid flow and heat transfer in an industrial continuous furnace

Abstract We present a three-dimensional computational study of the transient heat transfer and turbulent fluid flow inside an arc-welding electrode continuous furnace. The model is implemented in FLUENT, a finite volume commercial code. Large difference in the length scales of the electrodes and the furnace, movement of the electrodes, and existence of various modes of heat transfer are the major factors influencing the accuracy and efficiency of the simulation. Two modeling strategies are used to overcome these difficulties. First, the electrode geometry and material composition are simplified using a thermally equivalent model. Second, the electrode movement inside the furnace is avoided by implementing time-dependent boundary conditions applied to a fixed domain. A fairly close agreement is obtained in comparing the load’s temperature history against the experimental data with an absolute relative difference below 2.7%. The space between the electrode trays in the furnace is very limited. The analysis shows that the input air does not circulate effectively between the trays. This lowers the thermal efficiency of the furnace and leads to uneven treatment of the electrodes. The present study provides guidelines to improve future furnace designs.

Contact line region heat transfer mechanisms for an evaporating interface

Many recent studies have emphasized the significance of contact line region heat transfer in applications such as boiling, thin film evaporation and enhanced heat transfer using wicking structures. Literature on heat transfer in the contact line region have focused on the adsorbed layer and transition region of the contact line. However, the region beyond the thin film region of the contact line plays a significant role in heat transfer and associated phase change at the dynamic interface. In this paper, heat transfer in the adsorbed and transition regions as well as in the macro region is reviewed. The need for studying the microconvective effects for polar liquids because of their broad practical application in boiling is also highlighted. Experimental and analytical works on the contact line region are explored and a comprehensive picture of its physical and heat transfer characteristics is presented. Various optical and thermal measurement techniques employed by researchers to understand evaporation in the contact line region are also reviewed. Finally, the contribution of the various modes of heat transfer within the contact line region during nucleate pool boiling is discussed and future research needs are identified.

Heat transfer search (HTS): a novel optimization algorithm

In this paper, a new metaheuristic optimization algorithm based on the law of thermodynamics and heat transfer is introduced. In the proposed algorithm, the search agents are molecules of the system that interact with each other as well as with the surrounding to attain thermal equilibrium state. The interactions of molecules are through various modes of heat transfer (i.e. conduction, convection and radiation). The performance of the proposed algorithm is investigated by implementing it for the parameter optimization of 24 well defined constrained optimization problems of Congress on Evolutionary Computation 2006 (CEC 2006). The results obtained using the proposed algorithm are compared with the results of some well-known metaheuristic search algorithms available in the literature. The statistical analysis of the experimental work has been carried out by conducting Friedman's rank test and Holm post hoc procedure. The comparative results validate the competitive and efficient performance of the proposed algorithm for solving constraint optimization problems.

Energetic performances of an optimized passive Solar Heating Prototype used for Tunisian buildings air-heating application

This paper deals with the energetic performances of a Solar Heating Prototype (SHP) conceived in our laboratory to prevail the Tunisian households’ air-heating needs. The conceived SHP mainly consists of a flat-plate solar collector, solar hot water tank and an active layer integrated inside a single room. Firstly, a complete model is formulated taking into account various modes of heat transfer in the SHP by means of the TRNSYS simulation program. To validate the TRNSYS model, experimental tests under local weather conditions were performed for 2days spread over 2months (March and April 2013). Predicted results were compared to the measurements in order to determine the accuracy of the simulation program. A parametric study was then achieved by means of the TRNSYS program in order to optimize SHP design parameters (Collector area, collector mass flow rate, floor mass flow rate, storage tank volume and thickness of the active layer). The optimization of all design parameters shows that to achieve a maximum performances from the SHP it is essential to use a solar collector with an area equal to 6m2 area, a collector mass flow rate equal to 100kgh−1 and a hot water storage tank with a capacity equal to 450l. Concerning the floor heating, the optimal values of mass flow rate and the active layer thickness are 200kgh−1 and 0.06m, respectively. The long-term SHP performances were afterward evaluated by means of the Typical Meteorological Year (TMY) data relative to Tunis, Tunisia. Results showed that for an annual total solar insolation of about 6493.37MJm−2 the average solar fraction obtained is about 84%. The results show also that the request of auxiliary energy is limited to the cold months of the year chiefly from December to Mars. The results show also that the SHP reduce the relative humidity inside the monozone room of about 40%.

Design of a chilled-water storage unit for solar air-conditioning

Solar powered air-conditioning requires some form of thermal energy storage in order to continuously balance energy supply and demand. The current paper is concerned with energy storage in the form of stored chilled water produced by a chiller during times when supply is more than demand. A new multi-tank storage unit design is presented and evaluated employing a specially developed mathematical model and computational code that predicts the various modes of heat transfer occurring within the storage unit. The computational code is both efficient and accurate; it is presented and employed to investigate the effect of various operating parameters and design options on performance. It is revealed that employing only modest insulation, the daily heat penetration is around 2% of the daily stored cooling-capacity. This loss is much lower than the losses involved in other forms of energy storage, such as battery storage, and expected to be cheaper and more environmentally friendly.

Long-term performances prediction of an evacuated tube solar water heating system used for single-family households under typical Nord-African climate (Tunisia)

This paper presents a year round energy performance monitoring results of a new type of domestic solar water heating system (DSWH) based on evacuated tube collector (ETC) recently commercialized in Tunisia. This investigation is performed according to two-folded approaches. Firstly, a complete model is formulated taking into account various modes of heat transfer in the DSWH by using TRNSYS simulation program. To validate the TRNSYS model, experimental tests under local weather conditions were performed for 6days spread over 2months (November and July 2010). Predicted results were compared to the measurements in order to determine the accuracy of the simulation program. The long-term system performance was then evaluated by means of the Typical Meteorological Year (TMY) data relative to Borj Cedria, Tunisia. Results showed that for an annual total solar insolation of 5489.3MJ/m2, a total of 4653.13MJ/m2 were collected by the 3.4m2 ETC. For 1047.8MJ/m2 of auxiliary energy supplied to the ETC DSWH system the average solar fraction was 84.4%. An energy performances comparison on yearly basis between the most commercialized flat-plate collector (FPC) DSWH system in Tunisia and ETC DSWH system is also presented in this paper. Results show that the annual average solar fraction of the FPC DSWH systems is lesser (68%) than that of ETC DSWH systems (84%) respectively. It is also found that ETC generated about 9% more energy than the FPC. An economic appraisal was also performed to select the most cost saving between the two DSWH systems.

Heat transfer characteristics of HiTAC heating furnace using regenerative burners

The aim is to experimentally study the various modes of heat transfer and to investigate the effect of the HiTAC flame characteristics on the heat transfer intensity and uniformity inside a semi-industrial test furnace using various industrial regenerative burners and various flame configurations namely; single-flame, twin-flame counter, twin-flame parallel and twin-flame stagger. Measurements of local instantaneous and average temperatures, heat fluxes and gas composition at several locations inside the furnace were carried out. It was observed that the HiTAC flame with highly reduced temperature fluctuations, turbulent intensity and combustion intensity have a larger reaction zone than a conventional flame. This large flame emits more thermal radiation in spite of its uniform and reduced temperature. Furthermore, the convective heat transfer was found to be uniform and as high as 30% of the total heat transfer to an object surface in the furnace. On the other hand, the very high reduction of NO x emission is a consequence of the low temperature and temperature fluctuation levels of the HiTAC flames. The above findings are valid to a similar extent in all burners and configurations but to less extent in the twin-flame counter configuration.

Modeling of Combustion and Heat Transfer in an Iron Ore Sintering Bed with Considerations of Multiple Solid Phases

The iron ore sintering was characterized as a relatively uniform process of solid material, coke combustion, various modes of heat transfer, and the complicated physical changes of solid particles. The sintering bed was modeled as an unsteady one-dimensional process of the solid materials with multiple solid phases, which confers a phase on each kind of solid material. Each solid phase had a specific particle size and compositions. Drying, condensation, coke combustion, limestone decomposition, generation of the macroscopic internal pore and shrinking of the bed were considered. Complicated modes of heat transfer including conduction, convection and radiation were considered. Numerical simulations of the condition in the iron ore sintering bed were carried out for various parameters: coke contents and air suction rates, along with some other parameters of the model. Calculation results were compared with the results of the sintering pot test. The temperature profiles and gas compositions showed a good agreement with the experimental data.

ExHFT for fourth generation heat transfer technology

This review paper considers the many techniques that have been developed to enhance convective heat transfer. A summary is given of the techniques that are effective for the various modes of heat transfer. Of particular interest currently is compound enhancement that involves the simultaneous application of several techniques, so as to produce an enhancement that is larger than the individual techniques operating separately. This is considered fourth generation heat transfer technology. To realize this, there are many experimental challenges to be overcome that relate to obtaining and applying data.

The Implications and Challenges of Enhanced Heat Transfer for the Chemical Process Industries

This paper considers the many techniques that have been developed to enhance convective heat transfer, with the emphasis on heat exchangers found in the chemical process industries. A summary is given of the techniques that are effective for the various modes of heat transfer. Of particular interest currently is compound enhancement that involves the simultaneous application of several techniques, so as to produce an enhancement that is larger than the individual techniques operating separately. There are many experimental and analytical challenges that relate to obtaining and applying data. The current advanced enhancement represents third generation heat transfer technology. This technology will find increasing application in all industries.

Individual heat transfer modes during contact baking of Indian unleavened flat bread ( chapati) in a continuous oven

Individual modes of heat transfer, i.e., conduction, convection and radiation were examined for their effects on baking Indian unleavened flat bread ( chapati) in a continuous oven developed in the institute. Mathematical models, which represent various modes of heat transfer, had been proposed. Different thermal parameters, e.g., specific heat, thermal conductivity and thermal diffusivity of chapati were determined. To obtain desired puffing, it is necessary to get impermeable layer. For this, the major portion of heat had to be transferred by conduction (66.73%). The combined contribution due to radiation was 24.97%, while the share of free convection was only 8.24%. The contribution due to radiation from the flame was negligible.

Modelling the performance of a large area plastic solar collector

A mathematical model for simulating the performance of a large area plastic solar collector was developed. The collector was installed and used to supply hot water to a hotel in Almeria, Spain. It consists of three main components, namely a plastic water bag, a UV-stabilized plastic sheet cover and an insulated floor. The plastic materials were used in this collector in order to reduce the investment cost. To develop the model, the various modes of heat transfer in the collector were analysed. A system of equations representing the model was simultaneously solved using the implicit finite difference method. The data obtained from the experiments were used to validate the model. It was found that the outlet water temperature calculated using the model agreed well with the experimental data. The model was then used to investigate the effect of various parameters on the performance of the collector. The results were used as a guideline to improve the performance of the existing collector. According to the investigation, the original design of the total collector length of 48.2 m was too long for the operational flow rate of 0.083 kg/s. From the simulation, the new values of the water depth and the flow rate were also recommended in order to increase the efficiency of the collector. The time lags between the radiation peak and the outlet water temperature peak were relatively large for all cases. It was also found that wind speed slightly affects the outlet water temperature.

Mixed convective heat and moisture transfer from a horizontal furry cylinder in a transverse wind

Free and forced convection heat and moisture transfer are investigated for the fur layer on a model animal limb or torso in a transverse wind. The analysis includes various modes of heat transfer through dry fur: conduction through the hairs, conduction, free and forced convection in the interstitial and external air, and evaporation from the skin. The relative contributions of these mechanisms are investigated by finite-element calculations for several mammalian species. Comparisons are made with the experimental data of Gebremedhin on Holstein calves.

Modeling of fluid flow and heat transfer in the plasma region of the dc electric arc furnace

A mathematical model describing the transport processes in the plasma arc in dc electric arc furnaces has been developed. The equations of conservation of mass, momentum, and energy are solved numerically in conjunction with Maxwell's equations of the electromagnetic field to calculate the velocity and temperature distributions in the plasma region. The heat transfer from the arc to a rigid anode surface is calculated. The model is applied to obtain quantitative results on the relative importance of the various modes of heat transfer from the electric arc to the anode surface. Computational results were obtained for varying arc current magnitudes and anode-cathode distances. The model predicts higher arc jet velocity and a broader arc core at higher arc current. The shorter arc length is more efficient for transferring heat to the anode.

Combined conduction, convection and radiation in a slot

This paper reports the results of a numerical study of the fundamental problem of combined mode heat transfer in a vertical slot. The paper is a sequel to the work of the authors on problems of interaction of natural convection with radiation and conduction in closed and open cavities. The present paper considers the intractions between all three modes of heat transfer and, hence, represents the most general case of interaction. The slot simulates a typical intermediate fin in a fin array or a typical section in an extruded heat sink commonly employed in practice. A detailed parametric study has led to useful correlations. The nature and extent of the coupling between the various modes of heat transfer are also elucidated.