Simple Heat Transfer Model Research Articles

Evaluation of thermal energy storage and recovery for an electrical energy mediator system

Energy storage both electrical and thermal is a rapidly emerging field of interest toward the development of more sustainable energy systems. The inherent inefficiencies associate with electrical storage can be partially overcome when thermal storage that collects and storage the waste thermal energy for alternative uses is integrated. Consequently, thermal energy storage systems are an enabling technology that will allow increased energy efficiency of a community, permit load levelling to reduce peak electricity demand. In order to facilitate a technology evaluation, a sizing strategy is developed for a phase change material (PCM) thermal storage system that determines system requirements under given thermal energy capture and recovery cycles. The sizing process utilizes a simplified one-dimensional heat transfer model that estimates melt times for a phase change material thickness without detailed geometry information. This melt time estimate allows the proportion of phase change material to fluid routing materials to be calculated, giving an estimate of material cost for the thermal storage cell to determine economic feasibility. The model is compared to both experimental data and computational fluid dynamics models in order to determine its limitations. Through a specific example of hydrogen based distributed electrical energy mediator system, the utility of the sizing model in determining the estimated cost of thermal energy storage is demonstrated.

Glass fiber reinforced rigid polyurethane foams

Rigid polyurethane foam (RPUF)/glass fiber composites have been fabricated from glass fiber, polymeric 4,4′-di-phenylmethane diisocyanate (PMDI) and polypropylene glycols (PPG) using HFC 365mfc as blowing agent. Thermal conductivity, glass transition and decomposition temperatures as well as the mechanical strengths of the foam increased with the addition of glass fiber. This indicates that an optimum fiber content should depend on the balance between the mechanical reinforcement and thermal insulation.The results were interpreted in terms of cell size, closed cell content, density, fiber dispersion and a simple series model for heat transfer of the composite foam.

Wake Flow and Heat Transfer Due to a Spherical Viscous Droplet

A numerical study on the buoyancy-assisted flow and heat transfer from a liquid spherical droplet falling in fluid medium is made. The investigation is based on the solution of the Navier-Stokes equations together with the energy equation inside and outside the droplet, along with a suitable interface condition. The governing equations for three-dimensional flow and heat transfer are solved through the pressure correction based iterative algorithm, SIMPLE. The Reynolds number for the exterior flow is considered below 300 with the Richardson number in the range 0 ≤ Ri ≤ 1.5. The form of the wake due to the viscous droplet and its influence on heat transfer and drag coefficient are analyzed for a wide range of physical parameters. It is found that by increasing the Reynolds number, the predicted rate of heat transfer is significantly increased for a liquid droplet compared to a solid sphere. The increment of viscosity of the droplet increases the drag experienced by the droplet but reduces the rate of heat transfer. An increase in Richardson number produces an increment in drag coefficient as well as in heat transfer. In order to establish a simplified model for heat transfer due to a viscous droplet, we compared our computed solutions with several empirical correlations for conjugate heat transfer and proposed a model (in absence of buoyancy). We have also investigated the validity of several empirical correlations for the drag coefficient.

Coke yield and heat transfer in reaction of liquid–solid agglomerates of Athabasca vacuum residue

AbstractThe ultimate coke yield was determined for agglomerates of Athabasca vacuum residue and coke particles by heating on Curie‐point alloy strips in an induction furnace at 503 and 530°C until all toluene‐soluble material was converted. Coke yield results from agglomerates were compared to the coke yield results from reacting thin films of vacuum residue. The average coke yield from the agglomerates was 23%, while the coke yield from thin films of 20 µm thickness was 11%, which supports the role of mass transfer in coke formation reactions. The ultimate coke yield was insensitive to vacuum residue concentration, agglomerate size, and reaction temperature. The temperature profile within agglomerates was measured by implanting a thermocouple, and a simple heat transfer model was used to describe the temperature variation with time. The effective thermal diffusivity of the agglomerates was 0.20 × 10−6 m2/s. Control experiments on reaction in thin liquid films confirmed that heating rates in the range of 14.8–148 K/s had no effect on the ultimate yield of coke yield.

The effect of increased pressure on interfacial heat transfer in the aluminium gravity die casting process

Contraction and distortion of a casting during cooling within a mould can force their respective surfaces together, with the associated increased interfacial pressure resulting in increased interfacial heat transfer. This problem has been examined for the case of gravity and low pressure die casting of an Al alloy, where an insulating coating is applied to the die cavity to assist filling of the mould. The degree of interfacial pressure was estimated to be, for a typical small die casting, at most about 21 MPa. Repeated applications of a compressive load showed that a freshly applied die coating became thinner and smoother, until a stable situation was reached after about ten applications. The interfacial heat transfer coefficient was estimated to be increased by about 20%, with an increase in the applied pressure by a factor of two, from 7 MPa to 14 MPa, and increased by about 40%, with an increase in the applied pressure by a factor of three, from 7 MPa to 21 MPa. The heat transfer mechanisms between the casting and the die surfaces were evaluated to produce a simple model of interfacial heat transfer which included conduction through the points of actual contact, in parallel with conduction through the interfacial gas between the points of actual contact, both mechanisms being in series with the heat transfer by conduction through the die coating. Evaluation of the model produced agreement with experimentally determined values of the interfacial heat transfer coefficient to within about 15%.

Thermal analysis of the defrost cycle in a domestic freezer

This paper presents a thermal analysis of a defrost cycle in order to design more efficient defrosting mechanisms in household refrigerators and freezers. A simple heat transfer model has been developed to determine energy flows from a defrost heater across various components of a refrigerator/freezer. The study measures power consumption and temperatures of a single temperature vertical empty freezer (in normal operation with and without defrosts) to determine the heat distribution from the radiant type electric defrost heater and its effect on power consumption. The surface temperature of the defrost heater was measured to be 520 °C for minimal frost and 560 °C for heavy frost. The efficiency of a defrost heater was measured to be 30.3%, while power consumption of the freezer was found to increase by 17.7% due to automatic defrost.

Prediction of human thermophysiological responses during shower bathing

This study develops a model to predict the thermophysiological response of the human body during shower bathing. Despite the needs for the quantitative evaluation of human body response during bathing for thermal comfort and safety, the complicated mechanisms of heat transfer at the skin surface, especially during shower bathing, have disturbed the development of adequate models. In this study, an initial modeling approach is proposed by developing a simple heat transfer model at the skin surface during shower bathing applied to Stolwijk's human thermal model. The main feature of the model is the division of the skin surface into three parts: a dry part, a wet part without water flow, and a wet part with water flow. The area ratio of each part is decided by a simple formula developed from a geometrical approach based on the shape of the Stolwijk's human thermal model. At the same time, the convective heat transfer coefficient between the skin and the flowing water is determined experimentally. The proposed model is validated by a comparison with the results of human subject experiments under controlled and free shower conditions. The model predicts the mean skin temperature during shower fairly well both for controlled and free shower bathing styles.

Development of a safety analysis code for molten salt reactors

The molten salt reactor (MSR) well suited to fulfill the criteria defined by the Generation IV International Forum (GIF) is presently revisited all around the world because of different attractive features of current renewed relevance. The MSRs are characterized by using the fluid-fuel, so that their technologies are fundamentally different from those used in the conventional solid-fuel reactors. In this work, in particular, the attention is focused on the safety characteristic analysis of the MSRs, in which a point kinetic model considering the flow effects of the fuel salt is established for the MSRs and calculated by developing a microcomputer code coupling with a simplified heat transfer model in the core. The founded models and developed code are applied to analyze the safety characteristics of the molten salt actinide recycler and transmuter system (MOSART) by simulating three types of basic transient conditions including the unprotected loss of flow, unprotected overcooling accident and unprotected transient overpower. Some reasonable results are obtained for the MOSART, which show that the MOSART conceptual design is an inherently stable reactor design. The present study provides some valuable information for the research and design of the new generation MSRs.

Temperature prediction in domestic refrigerators: Deterministic and stochastic approaches

A simplified steady state heat transfer model was developed for a domestic refrigerator (without a fan). This model considers circular airflow, heat exchange by natural convection between the air and the cold/warm walls and between the air and the load. Radiation between cold/warm walls and load is also taken into account. The model considers the temperature variation related to the height of the refrigerator (top, bottom) and the position (near the cold wall, near the warm wall). Two random parameters were considered: the room and thermostat temperatures. These values were then introduced into the model enabling the calculation of the load and air temperatures. Analysis of the predicted temperatures was undertaken using comparison with survey data; good agreement was obtained for the mean value and the standard deviation. This model could prove to be useful in the development of a risk evaluation tool.

On possibility of vapor-phase combustion for fine aluminum particles

A simplified heat transfer model applicable for vapor-phase combustion of individual fine metal particles predicts existence of a critical particle diameter, below which the vapor-phase flame alone cannot be self-sustaining. Other heat generation mechanisms (i.e. surface oxidation) should complement the vapor-phase flame. The predicted critical particle diameter is a function of the flame temperature and pressure. For single aluminum particles burning in atmospheric pressure air, CO 2 and H 2O, the predicted critical particle diameters are close to 6, 7, and 15 μm, respectively.

Influence of Casting Parameters on the Critical Casting Size of Bulk Metallic Glass

The effect of various casting parameters and thermophysical properties of bulk metallic glasses (BMGs) on critical section size, defined as the maximum casting size that generates a through-section amorphous alloy, has been analyzed using a simple heat-transfer model. It was found that the interfacial heat transfer between the mold and the casting has a strong influence on the critical section thickness of a BMG. It is argued herein that the critical cooling rate rather than the critical casting size is a more robust indicator of glass-forming ability (GFA). Further, with respect to the critical cooling rate and heat-transfer effects during casting, a distinct difference was found between the critical casting thickness in rectangular-shaped castings and the critical casting diameter in rod-shaped castings, and a relationship was derived for relating these parameters.

A simplified model of heat and moisture transfer for tailrace tunnel ventilating in hydropower station

Tailrace tunnel ventilating can be used as a highly efficient and energy saving air conditioning for factory buildings in hydropower stations. This paper describes a simplified model of heat and moisture transfer for tailrace tunnel ventilating system with the assumption of isothermal boundary for tunnel wall, and constructs a small-scale experimental set-up to simulate the heat and moisture transfer process during the air flowing through the tunnel. The model has been validated against the experimental and field-test data, and it shows a good agreement. This simplified model is intended for use in annual performances prediction of heat and moisture transfer or provides the design tools for tailrace tunnel ventilating system in hydropower stations.

A new predictive dynamic model describing the effect of the ambient temperature and the convective heat transfer coefficient on bacterial growth

In this study, predictive microbiology and food engineering were combined in order to develop a new analytical model predicting the bacterial growth under dynamic temperature conditions. The proposed model associates a simplified primary bacterial growth model without lag, the secondary Ratkowsky “square root” model and a simplified two-parameter heat transfer model regarding an infinite slab. The model takes into consideration the product thickness, its thermal properties, the ambient air temperature, the convective heat transfer coefficient and the growth parameters of the micro organism of concern. For the validation of the overall model, five different combinations of ambient air temperature (ranging from 8 °C to 12 °C), product thickness (ranging from 1 cm to 6 cm) and convective heat transfer coefficient (ranging from 8 W/(m 2 K) to 60 W/(m 2 K)) were tested during a cooling procedure. Moreover, three different ambient air temperature scenarios assuming alternated cooling and heating stages, drawn from real refrigerated food processes, were tested. General agreement between predicted and observed bacterial growth was obtained and less than 5% of the experimental data fell outside the 95% confidence bands estimated by the bootstrap percentile method, at all the tested conditions. Accordingly, the overall model was successfully validated for isothermal and dynamic refrigeration cycles allowing for temperature dynamic changes at the centre and at the surface of the product. The major impact of the convective heat transfer coefficient and the product thickness on bacterial growth during the product cooling was demonstrated. For instance, the time needed for the same level of bacterial growth to be reached at the product's half thickness was estimated to be 5 and 16.5 h at low and high convection level, respectively. Moreover, simulation results demonstrated that the predicted bacterial growth at the air ambient temperature cannot be assumed to be equivalent to the bacterial growth occurring at the product's surface or centre when convection heat transfer is taken into account. Our results indicate that combining food engineering and predictive microbiology models is an interesting approach providing very useful tools for food safety and process optimisation.

Thermoengineering analysis of feasibility of heating roads by low-potential heat

A relatively simple model of heat transfer in systems intended to heat a roadbed is proposed. A program for calculating temperature fields has been developed. Variant calculations that make possible to assess the effect of various factors on the temperature of the road surface have been performed.

A Simplified Model of Heat and Mass Transfer Between Air and Falling-Film Desiccant in a Parallel-Plate Dehumidifier

This paper presents a simplified model to predict the heat and mass transfer between air and falling-film liquid desiccant during dehumidification in a parallel-plate absorber. First-order, ordinary differential equations are used to estimate the heat and mass transferred, and explicit equations are derived from conservation principles to yield the exiting absorber conditions for different flow arrangements. The developed model uses a control volume approach that accounts for the change in film thickness and property values. The model results were within 5% of a more complicated parallel-flow model currently available in literature. The model was also in good agreement with existing experimental data for a counterflow absorber.

Evaluation of the Impact of Potential Fire Scenarios on Structural Elements of a Cable-Stayed Bridge

It has become necessary to consider the potential exposure of major bridges to flames from oil or liquefied petroleum gas fires as part of comprehensive risk assessments being undertaken for major infrastructure. This article identifies and characterizes potential fire scenarios relevant for bridges and considers the likely impact on elements of a bridge structure. As part of this study, simplified heat transfer models are developed for the purpose of determining the temperatures of key structural steel elements. These models, which provide an efficient method for determining heat transfer through elements such as heavy bundled steel cable, are described in this article. The Newton—Raphson method is used to solve the key equations. Calculated temperatures are presented for the various fire scenarios involving key structural steel elements and the implication of these temperatures for the structural behavior of the steel elements is considered.

Thermal analysis of an historical Turkish bath by quantitative IR thermography

The study is focused on the use quantitative IR thermography (QIRT) for the examination of thermal performance of an historical Turkish bath (hamam). A simple heat transfer model was applied to achieve reference surface temperatures during the IRT surveys. The reference temperatures calculated and the real surface temperatures measured have been compared each other to better understand the thermal performance of historical structure in a non destructive way. Another issue was the failures evaluation in relation to the recent restoring interventions. The study was supported by microclimatic monitoring of indoor and outdoor conditions. The thermophysical properties of the historic materials were determined by laboratory analysis and the data were used for the heat loss calculations

A remining technology of underground coal gasification at Zhongliangshan Coal Mine

The field trail used a mixture of steam and air with various levels of oxygen enrichment. Steady conditions were achieved in the field trail which produced high quality hydrogen-enriched syngas. To understand and optimize the UCG process, a simplified heat and mass transfer model was presented, providing a predictive tool for temperature and the major constituents of the syngas production. The model is compared with the field trail measurements for air and two levels of oxygen enrichment, showing reasonable agreement for the channel temperature and product syngas concentration profile.

Drawback During Deposition of Overlapping Molten Wax Droplets

Two overlapping droplets impacting on a solid surface coalesce and recoil so that the edges of the droplets are drawn back, a phenomenon called drawback. A series of experiments were conducted on the merging of two overlapping wax droplets deposited on an aluminum drum to characterize the drawback process between the two droplets. Drum temperature, droplet overlap ratio, and the time interval between impacts of droplets were varied. Wax bumps, formed by coalescence of two droplets on the drum surface, were photographed and their length and width measured. An aspect ratio and dimensionless drawback index, quantifying the extent of drawback, were calculated from these measurements. When drum temperature is increased, or the time interval between impacts of the two droplets is reduced, there is more drawback and the ink bumps become round, since the cooling rate of droplets is slower and droplets have a longer time to change shape due to surface tension. A simple heat transfer model was developed to predict changes in droplet-cooling rate with changes in droplet overlap, substrate temperature, or time interval (deposition frequency). Experiments were also conducted on the formation of lines by depositing 20 droplets. Measurements on the drawback of two droplets were used to predict conditions under which broken lines are obtained.

Simplified Two-Dimensional Analytical Model for Friction Stir Welding Heat Transfer

A two-dimensional heat transfer problem pertaining to friction stir welding is developed by converting various pin tool configurations of interest to a simplified pin only configuration by assigning an equivalent heat flux to the pin surface. Mechanical dissipation heating, responsible for the welding, is modeled by means of a thermal boundary condition at the pin surface. A series solution is developed for the temperature distributions in the workpiece (assumed to be infinite) and these distributions are analyzed at various radial and circumferential locations. It is found that the closed form solutions developed are not influenced greatly by the truncation numbers of this series. Maximum reduced temperatures pertinent to configurations available in literature are estimated from the series and one term solutions developed and compared with those observed. Furthermore, the applicability of one term solution of this series is tested for various parametric combinations based on models available in literature. It is found that the one term solution can be applied to within a reasonable range of the process governing parameters.