Simple Heat Transfer Model Research Articles

Heat transfer analysis of blood perfusion in diabetic rats using a genetic algorithm

Diabetes is frequently associated with structural and functional impairment of the microcirculation. Blood perfusion is an important indicator of both physiological and pathological conditions of the microcirculation. Given that temperature is closely related to blood perfusion and is more easily measured, blood perfusion can be estimated from variations in skin temperature using an inverse method.The aim of this paper was to develop a thermal analysis method for estimation of blood perfusion and apply it in the assessment of skin blood perfusion in diabetic rats. First, diabetes was induced in the rat models of the experimental group. Skin temperature from the rats left hind paws was measured during a 10-min local heating period followed by a 15-min cooling period. A simple one-dimensional heat transfer model, including an arteriolar vessel node, was used to describe the skin heat transfer process. The blood perfusion of the arteriole was estimated by correlating the calculated skin temperature with known experimental temperatures using a genetic algorithm. The results indicated that the average blood perfusion in the control group was higher during local heating and decreased faster during the cooling period, showing dynamic responses to the thermal stimuli. In contrast, the blood perfusion of diabetic rats was reduced compared with that of the control rats during the heating phase and the rate of decrease in perfusion during the cooling stage was similarly reduced, implying a slower response to thermal stimulation in these rats. It is interesting to note that diabetic rats fed a normal diet showed a similar blood perfusion pattern to that in the control rats, implying that diet may be important in the treatment of diabetes-associated microvascular dysfunction.

Evaluation of a variable flow ejector for anode gas circulation in a 50-kW class SOFC

Solid oxide fuel cells (SOFC) are highly efficient in terms of converting hydrogen's chemical energy into electrical energy through electrochemical reactions and for generating power in the range of several kW to several tens of kW. A variable flow ejector equipped with an adjustable recirculating flow rate mechanism was designed for this investigation. A prototype was manufactured to control the circulation of anode exhaust gas for a 50-kW class SOFC system. The ejector performance was evaluated using SOFC simulator equipment that simulated the pressure and temperature environment of a 50-kW SOFC system. In the heating simulation experiment, the mass flow rate ratio of the driving gas to the suction gas could be controlled from 3.7 to 5.4 under conditions simulating 100% of the rated load operation and from 4 to 7.5 when simulating 30%–50% of the partial load conditions. A simple heat transfer model for the motive nozzle was used in the ejector analysis, and issues for improving the ejector recirculation performance in the high-temperature field were identified.

A simplified heat transfer model of a closed refrigerated display cabinet

Installation of doors on open refrigerated display cabinets is one of the simple and effective methods to improve the cabinet performance because it can reduce the warm and humid air infiltration into the cabinets. However, the presence of doors can change airflow pattern, which in turn influences the heat transfer and product temperatures. It is, thus, necessary to understand these thermal phenomena. This study proposes a simplified heat transfer model developed based on a zonal approach to describe the evolution of air and product temperatures at different zones in the closed refrigerated display cabinet. For model validation, the predicted values were compared with measured air and load temperatures for three external air temperatures. Good agreement was found between the predicted and measured temperatures with the maximum difference of less than 0.5 °C for every studied position and the overall mean absolute error of 0.2 °C. The model was then used to predict the effect of the air infiltration through the door gaps on the performance of the closed display cabinet in both thermal and energy aspects. This developed model will be further used in the modeling of the cold chain, where several types of refrigeration equipment are used to keep products under low-temperature conditions from production to consumption. Through integration of quality and microbiological predictive models, this approach can be used as a numerical tool for an evaluation tool for consumer risk.

Characterization of the thermal flux transferred by the plasma arc to the surface of the liquid bath in the Plasma Arc Melting Cold Hearth Refining process (PAMCHR)

Europe is a major provider of titanium scrap but until now there has been no european facility dedicated to the production of aeronautical quality titanium alloys by recycling. UKAD, a joint-venture between Aubert & Duval and UKTMP, ADEME and Crédit Agricole Center France, created the company EcoTitanium, where the recycling of machining scraps is achieved using the Plasma Arc Melting Cold Hearth Refining (PAMCHR) process with the aim of producing such alloy ingots. The determination of the heat flux transferred from the plasma column to the surface of the liquid bath is an important issue needed for the development of a future 3D process modeling based on Ansys-Fluent CFD software. With this preliminary aim of predicting the heat flux, a study coupling pilot furnace testing and numerical modeling has been performed. Melting tests were carried out in a pilot PAMCHR furnace in MetaFensch, with a stationary torch. In parallel, the different contributions of the heat flux transferred to the surface of the liquid bath were identified and implemented in a simple 2D axisymmetric heat transfer model. Numerical results are compared to the experimental measurements, in order to define the heat flux transferred from the plasma torch.

Reduction of Heat Losses Using Quadruple Heating Pre-Insulated Networks: A Case Study

The paper presents an analysis of heat loss and reductions of annual emissions of air pollutants of a quadruple pre-insulated heating network by comparing this solution with the existing pre-insulated network consisting of four pre-insulated single pipes and the variant consisting of two twin pipe pre-insulated. For calculations, an existing heating network located in central Poland was adopted, where heat is transported for heating purposes of buildings and domestic hot water with circulation of domestic hot water through four separate pre-insulated underground pipes. The idea of the construction of four pre-insulated pipes presented in the paper consists in the location of four steel pipes in a common round thermal insulation, which perform the role of heat transport for heating purposes in multi-family buildings (supply and return) and two pipes transporting hot water (a pipe with domestic hot water with circulation). In Poland, heating pipes used in multi-family housing have a larger diameter compared to domestic hot water pipes, which is why standard twin pipe heating pipes have been used in the construction of four pre-insulated networks, in which the domestic hot water pipe has been added to the thermal insulation and circulation of domestic hot water. In order to determine heat losses, a simplified two-dimensional model of conductive heat transfer was developed using Fortran to create a computer program. The results of numerical simulations show that the use of twin pipes for the construction of pre-insulated quadruple networks has contributed to a significant reduction in heat loss in relation to the existing single pre-insulated network (up to 57.1%), while reducing the thermal insulation field of the cross-section of the pre-insulated pipe by 21.4%.

Fabrication of bio-inspired non-fluorinated superhydrophobic surfaces with anti-icing property and its wettability transformation analysis

Ice accumulation leads to several inconvenient and even threatening situations in cold regions. This paper reports an efficient, non-toxic and low-cost method to fabricate hierarchical superhydrophobic surfaces on aluminum alloy with a water contact angle of 155° and a sliding angle of 5°. Tests were performed in an in-house apparatus to evaluate the ice adhesion strength and ice shear strength of the as-prepared surfaces and untreated aluminum surfaces. Results show that ice adhesion strength on different wettability surfaces strongly depends on the efficient specific surface area of the ice-solid interface. The ice shear strength was determined by different ice-solid interface contact shapes. Besides, the freezing delay time was recorded and different mechanisms of the icing process were analyzed by a specific simplified heat transfer model. The de-icing and self-cleaning test shows that the air cushion on the superhydrophobic surface can prevent the melted water from secondary icing and potential contaminations, especially at low temperature. Our results indicate that the as-prepared and recovered superhydrophobic surfaces present excellent anti-/de-icing properties and the special contact conditions and heat transfer process of the ice-substrate interface contribute to the anti-icing property of the superhydrophobic surface.

Acoustic Excitation as Triggering Mechanism of the “DC Shift” in Hybrid Rockets

A positive “DC shift” is the sudden rise of mean chamber pressure, which is often observed in hybrid rockets during unstable motor instances whereby the dynamic pressure signals show oscillations at both low frequency and acoustic frequency. The exact triggering mechanism of DC shift has not been understood yet; the main idea addressed in this paper is that the chamber pressure oscillations at the fundamental hybrid low frequency are triggered by the heat flux fluctuations occurring at a longitudinal acoustic mode of the chamber, and that those fluctuations are such that a net upward shift of regression rate is produced. A simple heat transfer model combined with the numerical solution of the thermal lag in the solid fuel is presented, which allows simulating the regression rate dynamics under mass and heat fluxes variations at the acoustic frequency. The computational results show that, by stimulating the thermal lag system with mass fluxes oscillating at acoustic frequencies, due to the boundary-layer delay time, the system is unstable, yielding fluctuations with the fundamental low frequency and a significant increase of the baseline steady-state value, whose magnitude depends on the time delay.

Optimal indoor heat distribution: Virtual heaters

It is well known that indoor heat distribution can affect energy consumption according to the thermal comfort of the occupants. While most work on this topic has focused on specific heaters and how they distribute heat, this paper proposes a new concept termed virtual heaters. Virtual heaters are a set of two optimized heat distributors that respectively maximize and minimize the energy consumption inside a room while maintaining the same level of thermal comfort. The maximum and minimum virtual heaters are then applied in a comparison with a real heater tested in a specific room at constant thermal comfort in order to quantify its ability to provide comfort while using a minimum amount of energy. To calculate the ”virtual heaters”, a simplified heat transfer model is formulated and implemented. A volumetric thermal comfort model using the predicted mean vote is also discussed and used. The ”simplified” heat transfer model with the thermal comfort constraint is then optimized via a sequential quadratic programming algorithm. The proposed method is applied to the heating of a room subject to an outdoor temperature of -20°C and compared to experimental results. Results show that the maximum virtual heater consumes approximately 35% more energy than the minimum virtual heater for the case considered herein.

Modelo reducido de un reactor nuclear y su utilización en las asignaturas del ámbito de la Ingeniería Nuclear

<p>En el grado de Ingeniería de la Energía, los alumnos pueden cursar las asignaturas de Tecnología Nuclear y Seguridad Nuclear. En ambas asignaturas se estudia el diseño de un reactor nuclear para que la central funcione en condiciones seguras. Para conseguir que los alumnos entiendan el funcionamiento de un reactor nuclear es interesante el uso de modelos matemáticos que reproduzcan tanto la potencia generada como la evolución de otras variables que afectan a la seguridad de la planta. En este trabajo, se presenta un modelo sencillo de reactor de agua a presión que permite observar la evolución temporal de la potencia del reactor y las temperaturas de los componentes más importantes de la planta. Para obtener la potencia generada se hace uso del modelo de cinética puntual, mientras que para el cálculo de las temperaturas del combustible y del refrigerante se utiliza un modelo sencillo de transmisión de calor. <br />De este modo, el problema se formula mediante un sistema de ecuaciones diferenciales ordinarias que se resuelve utilizando métodos numéricos. El desarrollo del modelo permite a los alumnos de Tecnología Nuclear y Seguridad Nuclear comprender la evolución temporal de alguna de las variables de un reactor y darse cuenta de los efectos estabilizantes que los parámetros de la termohidráulica tienen sobre la potencia generada.</p>

Experimental investigation on the plasmonic blended nanofluid for efficient solar absorption

A plasmonic blended nanofluid formed by mixing Au nanoparticles with different shapes in water is proposed in this paper for direct solar absorption. Optical and thermal properties of the plasmonic blended nanofluid are studied numerically and experimentally. Resonant characteristics of Au plasmonic nanoparticles are tuned by particle shapes. Compared with single-component nanofluid, the extinction spectrum of this plasmonic blended nanofluid is broadened. The matching of extinction spectrum and solar radiation spectrum is tuned by adjusting the proportion of components in blended nanofluid. Photothermal properties of three types of plasmonic blended nanofluids are measured experimentally. Due to the higher extinction coefficient, the blended nanofluid exhibits higher temperature rise. A simplified heat transfer model is established to verify experimental results. Simulation results are consistent with experimental results. Before experiment, photothermal properties of different nanofluids can be qualitatively compared by using the simulation model, which can effectively reduce the number and cost of experiments.

New Method for Calculating the Heating of the Conductor

The paper describes the core heating of ACSR (aluminum conductor steel—reinforced) conductor in stable operation under different environmental conditions. The calculations are greatly simplified in a steady state—we can calculate on a balance of power instead of a balance of energies. At a known surface, the temperature of the conductor due to solar radiation, natural convection, and joules heating as well as the temperature of the steel core were calculated, which is relevant for the tensile strength of the rope. Measurements of the surface of the conductor and the core rejected a simple model of heat transfer—it is also necessary to take into account empty air spaces between the wires of a rope. On the basis of measurements, a new model has given satisfactory compliance with the measured values.

Dynamic heat transfer modeling of a closed refrigerated display cabinet

This study proposes a simplified heat transfer model of a closed refrigerated display cabinet using a zonal approach. The dynamic model was developed and solved with a spectral method to predict the temperature fluctuations occurring with “on/off” compressor regulation. The equivalent load thickness in which the temperature fluctuations were of the same order as those at the load surface was estimated as a function of “on/off” compressor operating frequency. The air temperature fluctuations are dampened whenever the air exchanges heat with solid surfaces (e.g. walls, loads) and with the ambient air (e.g. through the doors). Despite exposure to large air temperature fluctuations (up to 5 °C), the load surface temperature showed only small fluctuations (ΔTl ≤ 0.5 °C). Good agreement was found between the predicted and the measured temperature fluctuations. The model can also be applied in order to predict air and load temperature increases during defrost operation.

Investigating temporal variation in the apparent volume fraction measured by time-resolved laser-induced incandescence

In many time-resolved laser-induced incandescence (TiRe-LII) experiments, it is common practice to relate the intensity emitted by laser-heated nanoparticles to the detected LII signal through a factor (here called the intensity scaling factor, ISF) that includes the particle volume fraction and other parameters that may not be the focus of the analysis. While, in the absence of evaporation or sublimation, the ISF should theoretically remain constant with respect to time, recent multi-wavelength measurements show that, in reality, it may vary with both time and fluence. We consider four candidate effects that contribute to this behavior: particle annealing; polydispersity in the nanoparticle-size distribution; background luminosity due to emission from nanoparticles in the line-of-sight before and behind the probe volume; and the temporal resolution of the detector. We demonstrate these effects by simulating TiRe-LII data for in-flame soot at atmospheric pressure, using new simplified heat transfer and annealing models. Analysis of experimental signals collected from flame-generated soot at atmospheric pressure reveals trends in the ISF similar to those predicted by simulations. These temporal variations provide important insights that can help to diagnose problems in TiRe-LII experiments and improve TiRe-LII models.

Investigation on transient energy consumption of cold storages: Modeling and a case study

It is believed that large cold storages account for approximately 80% of overall cold storage volumes in China. Refrigeration energy demand of the buildings represents over 60% of energy consumption and thus users have considerable incentive for decarbonization by optimizing the refrigeration energy supply. This fact makes transient predictions of energy consumption and thermal modeling of the buildings become important. In this study, transient energy consumption of cold storages is mainly analyzed. A simplified 3R2C heat transfer model is used to simulate the cooling energy dissipate from building envelop. A semi-empirical equation is applied to simulate infiltration cooling load. The physical model of the internal mass is represented and calibrated by data-driven modeling approach with detailed transient simulation of the other two parts and measurement data. The overall model is validated by the measurement input power of a large cold storage as a case study. The modeling process show that the variety of cooling load of the internal mass is linear dependent on the variety of room temperature. The simulated results of the overall model have a good agreement with the measurement input power (RN_RMSE = 4.95%, R2 = 0.92, CVRMSE = 5.27% and MBE = 0.48%).

Integrated solar still with an underground heat exchanger for clean water production

A simplified model of heat and mass transfer of integrated solar still with an underground heat exchanger (GHEX) is presented. The solar still has 70 m length and 0.5 m2 inlet area. Moreover, the underground heat exchanger was 70 m length and 0.2 m diameter. The model includes a theoretical analysis in the transient mode for both the solar still and underground heat exchanger under warm climate region throughout the year. This system uses solar energy to saturate the air in the solar still, which is conducted into a buried pipe. Fresh water will be formed by condensation as long as the surface temperature of the pipe wall is less or equal to the dew point temperature of the humid air. A numerical method has been employed to solve the governing equations of heat and mass transfer characteristics in both the solar still and the underground heat exchanger. The soil temperature was taken as a function of soil depth and time along the year. The amount of fresh water produced was found strongly depends on solar intensity. It was respectively 4.7 (kg/m day) and 0.32 (kg/m day) for the highest and lowest values of solar radiation during the year. A good agreement has been shown when the results were compared with previous work.

A model-based method to control temperature and humidity in intermittently heated massive historic buildings

The indoor climate of historic buildings is governed not only by human comfort but also the desire to preserve these buildings and their interiors. For preservation, relative humidity is the most important parameter, including its amplitude and change rate. To control the change rate of relative humidity at a heat-up event for an intermittently heated massive historic building with heavy masonry walls, a simplified model for heat and moisture transfer at the heat-up time is presented. A method to derive the time constants and hygrothermal parameters of the building from measurements taken during a step response test is proposed and validated. The model with its parameters can be used to predict both heat-up time to reach the target temperature as well as the amount of moisture gained from the walls. These predictions are in turn used to calculate the decrease in relative humidity during the heat-up event. The indoor air relative humidity is predictable as it is determined by the air temperature and air mixing ratio. A control algorithm that uses the model for predicting and controlling the change rate of relative humidity by shaping the heating power is presented and validated using simulation results.

A new simplified model for condensation heat transfer of zeotropic mixtures inside horizontal tubes

Simple and accurate prediction models are highly demanded for industrial design of heat exchangers using refrigerant mixtures. This paper presents some data taken during condensation of R32/R1234ze(E) and proposes a new method to predict condensation heat transfer coefficient of zeotropic mixtures inside horizontal tubes. The method is developed based on the heat transfer model by Cavallini et al. (2006) for pure fluids by including additional heat transfer resistances during zeotropic mixtures condensation. The heat transfer resistances are modified from the Bell and Ghaly (1973) method by considering the effect of interfacial roughness and non-equilibrium phenomena, following the approach by Del Col et al. (2005). This new model is easy to implement and requires low computational effort. The predicted results by the model show a satisfactory agreement with experimental data covering various mixtures and a wide range of working conditions.

Experimental and Analytical Characterization of Firebrand Ignition of Home Insulation Materials

Wildland firebrands are known to ignite materials in attic spaces of homes. To clarify the effects of choices in attic insulation materials for homes located at the wildland urban interface, this study seeks to characterize the effects of firebrand characteristics on the ignition propensity of several common insulation materials: polyurethane foam, expanded polystyrene (EPS), extruded polystyrene (XPS), flame retarded and non-flame retarded denim, and flame retarded and non-flame retarded loosefill cellulose. An experimental system was developed to explore the effects of firebrand heating, air flow, and firebrand configuration on ignition. For an equal initial mass of wooden material, two firebrand configurations were generated: a single whole firebrand and multiple (five) fragmented firebrands. Relative to whole firebrands, the fragmented firebrands were found to more reliably ignite the insulation materials. Thermoplastic insulation material would only ignite in a temporary flash flame, but did not support sustained burning. Following the flash flame, the firebrands would melt through the synthetic polymer material (XPS and EPS) and cease smoldering. Cellulosic insulation materials would ignite in a sustained fire provided that there was adequate air flow. A simple heat and mass transfer model was developed to describe the ignition process due to firebrand deposition. Traditional lab-scale experiments, thermogravimetric analysis and cone calorimetry, were performed to parameterize the model. Results followed experimentally observed firebrand temperature patterns. There was an average error of approximately 8.5% between firebrand temperature model predictions and experimental measurements. Also, consistent with the experimental results, the model predicted that increasing air flow increased ember temperature and reduced the time to ignition for cases in which ignition occurs.

Numerical simulation of a simplified, transient, 2D, non-reactive heat transfer model of a lab-scale fixed-bed pyrolysis reactor

Pyrolysis of agro-industrial residues can be used to produce alternative liquid fuels to reduce consumption of fossil fuels. To study these biomass pyrolysis processes, experimentation and numerical simulations are commonly compared. For the latter, thermophysical properties of biomasses and proper thermal models of pyrolysis reactors are required in numerical simulations. This paper presents values for the specific heat capacity, density, and thermal conductivity of Colombian Coffee Husk, as well as a simplified non-reactive heat transfer model of a lab-scale, fixed-bed pyrolysis reactor. The thermal conductivity was determined by combining the axial rod method with transient temperature measurements and an inverse problem solution. CH properties values determined were 1.62 kJ/kg K, 1390 kg/m3, and 0.19 W/mK. To test the validity of the reactor’s thermal model simulation, temperature profiles results were compared with results from an experimental study employing a pyrolysis reactor with the same characteristics. Variations of temperature profiles in time were compared and good agreement between numerical and experimental results was observed for temperatures lower than pyrolysis temperatures (300–400 °C), validating the simplified model. Temperature differences above 300–400 °C were expected as chemical reactions were not included in the numerical model.

Major Issues in Simulating Some Arctic Snowpack Properties Using Current Detailed Snow Physics Models: Consequences for the Thermal Regime and Water Budget of Permafrost

AbstractAccurately simulating the physical properties of Arctic snowpacks is essential for modeling the surface energy budget and the permafrost thermal regime. We show that the detailed snow physics models Crocus and SNOWPACK cannot simulate critical snow physical variables. Both models simulate basal layers with high density and high thermal conductivity, and top layers with low values for both variables, while field measurements yield opposite results. We explore the impact of an inverted snow stratigraphy on the permafrost thermal regime at a high Arctic site using a simplified heat transfer model and idealized snowpacks with three layers. One snowpack has a typical Arctic stratification with a low‐density insulating basal layer, while the other (called Alpine‐type snowpack) has a dense conducting basal layer. Snowpack stratification impacts simulated ground temperatures at 5 cm depth by less than 0.3 °C. Heat conduction through layered snowpacks is therefore determined by thermal insulance rather than by stratification. Ground dehydration caused by upward water vapor diffusion is 4 times greater under Arctic stratification, leading to a larger latent heat loss, but also to a lower soil thermal conductivity caused by ice loss, so that the overall effect of dehydration on ground temperature is uncertain. Snowpack stratification is found to affect snow surface temperature by up to 4 °C. Lastly, different snow metamorphism rates lead to a lower Alpine snowpack albedo, contributing to a warmer ground. Quantifying all these effects is needed for adequately simulating permafrost temperature. This requires the development of a snow and soil model that describes water vapor fluxes.