Simple Heat Transfer Model Research Articles

Feasibility of Using Wood Chips to Regulate Relative Humidity Inside a Building: A Numerical Study

The use of bio-based materials in buildings has become more and more significant last years. In most of the cases, their health properties and natural provenance have made them a great solution to face global climate warming and the new policies to reduce building energy consumption. In many thermal problems, bio-based materials can allow to optimize the building thermal behavior according to its energy consumption and inside comfort conditions. So it is when they are used as an insulation material in the building. However, it is not the case in this paper. In fact, the bio-based matter is rather used as a desiccant wheel to control air conditioning inside the building. The aim of this paper is to numerically verify if it is possible to use a bed of wood chips as a hygroscopic material (or a desiccant matter) in order to modify the relative humidity inside the building in Reunion Island and so improve thermal comfort. A simple model of heat and mass transfer between a bed of wood chips and building inside air has been set up and implemented into a validated building simulation code named ISOLAB. Numerical simulations were set up for the four climate zones of the island regulations and a focus has been made on the low altitude one (with high, solar irradiation, temperature and relative humidity). Simulation results give the thermal behavior of the building particularly the temperature and relative humidity of inside air temperature, and temperature and moisture content of wood chips. The obtained results lead to determine if the wood chips bed is suitable for the reference building and to verify its technical feasibility (wood species, size of the bed, integration to the building, etc.). The results show that the use of a WCB help to decrease the building inside air temperature and water content up to 10°C less and 11.6 g.kg-1 less. These are the ways to improve inside comfort conditions. Indeed, comfort analysis have shown the possibility to significantly increase building users’ thermal comfort when coupled with a fan and natural ventilation, like the regulation needs for low altitude climate. In this case, a gain of 68% of year time is achieved for a building equipped with WCB system compared to one without it (6308 hours of comfort over a year with the WCB against 350 hours without WCB). So the WCB seems to be able to help reducing cooling loads in tropical climate conditions.

Comparisons of photovoltaic modules for their performances based on different substrates

Performance of photovoltaic (PV) module is investigated on different substrates, TPT, glass and aluminum. Theoretical simulations are made to calculate temperature field for those modules using a simple heat transfer model based on the finite element method (FEM). Theoretical results are obtained by keeping 3.2 mm of substrates thicknesses, and the average temperatures of PV modules are 62.78 °C, 64.38 °C and 68.71 °C for TPT, glass and aluminum substrates, respectively. Furthermore, the temperature distribution is studied with the specific substrate thickness, and interestingly only the temperature with aluminum substrate is decreased by increasing the substrate thickness. In addition, experiments are designed for the three modules, which are encapsulated and measured under non-constant and constant temperature conditions. Both theoretical and experimental results along with the temperature have a good agreement with each other. Power decays of three PV modules are also estimated, and found that their electrical efficiencies are close to 11%.

Topology optimization of channel cooling structures considering thermomechanical behavior

A topology optimization method is presented to design straight channel cooling structures for efficient heat transfer and load carrying capabilities. The optimization is performed on the structural cross section that consists of solid, void, and fluid coolant. A simplified convective heat transfer model is used to simulate the flow characteristic in the channels with a low computational cost. Besides, a continuous design-dependent surface-based penalty approach is proposed to ensure a meaningful inlet fluid temperature during the continuous process of the fluid topology alteration. Coupled thermomechanical problems are solved to account for the engineering requirements on the uniformity of temperature and structural deformation tolerance. Furthermore, a phase-interface constraint is implemented to prevent unrealistic boundaries that adjoin the liquid to the void or to the outer boundaries of a design domain. Numerical examples of designing a lightweight cooling support frame and a hot stamping tool structure subject to uniform or non-uniform thermomechanical loads are given to demonstrate its applicability. Verification results of 3D structures by a full-blown turbulent fluid simulation show that the proposed approach is effective in yielding channel cooling structures with optimized heat transfer capabilities and well-controlled structural deformation.

A simplified CFD model for spectral radiative heat transfer in high-pressure hydrocarbon–air combustion systems

Detailed radiation modeling in piston engines has received relatively little attention to date. Recently, it is being revisited in light of current trends towards higher operating pressures and higher levels of exhaust-gas recirculation (EGR), both of which enhance molecular gas radiation. Advanced high-efficiency engines also are expected to function closer to the limits of stable operation, where even small perturbations to the energy balance can have a large influence on system behavior. Detailed radiation modeling using sophisticated tools like photon Monte Carlo/line-by-line (PMC/LBL) is computationally expensive. Here, guided by results from PMC/LBL, a simplified stepwise-gray spectral model in combination with a first-order spherical harmonics (P1 method) radiative transfer equation (RTE) solver is proposed and tested for engine-relevant conditions. Radiative emission, reabsorption and radiation reaching the walls are computed for a heavy-duty compression-ignition engine at part-load and full-load operating conditions with different levels of EGR and soot. The results are compared with those from PMC/LBL, P1/FSK (P1 with a full-spectrum k-distribution spectral model) and P1/Gray radiation models to assess the proposed model’s accuracy and computational cost. The results show that the proposed P1/StepwiseGray model can calculate reabsorption locally and globally with less than 10% error (with respect to PMC/LBL) at a small fraction of the computational cost of PMC/LBL (a factor of 30) and P1/FSK (a factor of 15). In contrast, error in computed reabsorption by the P1/Gray model is as high as 60%. It is expected that the simplified model should be broadly applicable to high-pressure hydrocarbon–air combustion systems, with or without soot.

Continuous measurement of contact heat flux during minced meat grilling

Few studies concerning contact heating of food products were found in the literature despite the importance of this mode of heat transfer in many operations (grilling, pan-frying) and its drastic impact on product quality change during heating. An original heating device to measure continuously contact heat flux and heating surface temperature was therefore developed and applied to the contact heating of turkey meat with a heating surface at 128°C, 215°C and 255°C. Based on the experimental results obtained, a simplified heat transfer model was developed and allowed the calculation of the parts of the total energy received by the product (i) used to raise its temperature (ii) used to evaporate water exuding from its lower surface during heating and (iii) lost by heat exchange with the surroundings. In the experimental conditions covered by our experiments, the part of energy used to evaporate water exuding from the product was found to vary between 55% and 67% of the total energy received. The precise evaluation of the kinetics of variation of the contact heat transfer coefficient (quantifying the quality of thermal contact between the lower surface of the meat sample and the heating plate) was also made possible. At 215°C and 255°C, this coefficient ranged from 500 to 100 W⋅m−2⋅K−1 between the start and the end of the cooking, these values being in the same order of magnitude as those measured in previous studies concerning single-faced grilling of meat.

Model development and performance analysis of the nanofluid-based direct absorption parabolic trough collectors

In this paper, a simple one-dimensional heat transfer model for the nanofluid-based direct absorption parabolic trough collector (NDAPTC) was developed, which was quick to solve, easy to understand, and unnecessary to solve the complicated differential equations. In an actual system, the vacuum condition between the inner tube and glass cover may be changed gradually over time. Therefore, a continuous function for calculating the convective heat transfer in the annulus from near vacuum to atmospheric pressure was established. Combined with it, the derived heat transfer model was suited to evaluating the long-running thermal performance of a NDAPTC quickly. The heat transfer model for a conventional indirect absorption parabolic trough collector (CIAPTC) was also constructed, aiming at comparing the heat transfer processes of this two kinds of collectors. Both the heat transfer models were validated by the test data from the reference. Additionally, the effects of physical parameters, operational and weather conditions on the collector thermal performance were analyzed. It was shown that improvement of the glass transmittance and emittance was crucial to raise the collector efficiency, especially at a higher inlet temperature. The pressure change in the annulus affected the collector thermal performance greatly, thus it tried to avoid the vacuum damage. The collector efficiency was raised with the solar irradiance and ambient temperature increasing and wind velocity reducing, which was more sensible to these weather conditions at a higher inlet temperature and pressure in the annulus. The results in this paper were useful for guiding the collector improvements and engineering designs theoretically, and were beneficial for cleaner production in civil and industrial heat utilization further.

Techno-economic analysis of waste heat recovery systems for wet-cooled combined cycle power plants

Increasing ambient temperature negatively impacts the performance of natural gas combined cycle (NGCC) power plants. There have been multiple approaches to mitigate this performance reduction by chilling the compressor inlet air of the gas turbine, as well as by recovering the waste heat emanating from the power plant to generate additional power. In the present study, a detailed techno-economic assessment on the application of different types of waste heat recovery systems used to chill the compressor inlet air has been assessed. A simplified thermodynamic and heat transfer model is developed to predict the performance of an evaporatively cooled NGCC at varying ambient conditions. By taking typical meteorological year (TMY3) hourly weather data for two different locations – Los Angeles, California and Houston, Texas – the yearly output for a 565 MW plant is predicted at a 100% capacity factor. The feasibilities of different waste heat recovery (WHR) systems including a flue gas driven absorption chiller, a steam driven absorption chiller, and an electrically driven vapor compression chiller are assessed by calculating the levelized cost of electricity (LCOE) for each scenario. The results of the analysis showed that, for a fixed WHR system costs (i.e., $ per kWth), the system powered by flue gas generated the smallest LCOE, followed by the mechanically-driven vapor compression, steam-heated chiller, and, finally, the gas turbine exhaust chiller for both the locations at all COP combinations. The analysis also investigated the impact of fixed investments cost, and the flue gas system again yielded the smallest LCOE, while still yielding a lower LCOE than at baseline case over a wide range of COP and tolerable cooling costs for both locations.

Near-extinction behavior of smoldering combustion under highly vacuumed environment

The smoldering behavior of porous combustibles near the extinction limit was examined under a highly-vacuumed environment at various oxygen concentrations. The considered ambient conditions of the present work is 1.0–90 kPa in absolute pressure filled with a nitrogen–oxygen mixture. A 2-mm-rod-shaped incense stick is considered as the burning specimen. The specimen was located vertically and forced ignition was made at the top of the specimen to complete the following downward propagation. Two well-known burning modes were identified; such as flame spread (flaming) and smoldering (non-flaming) depending on the imposed ambient conditions. The results showed that the smoldering rate near the extinction limit decreased dramatically as the ambient pressure decreased. Temperature profiles inside the smoldering specimen were successfully measured by fine thermocouple even at the near-extinction limit. The results imply that there is a peak located slightly below the top surface. Based on the simple radiative-conductive heat transfer model, the role of radiation heat transfer for a forward temperature profile is discussed within the range for which oxidative reaction is neglected. It is implied that the radiant heat transfer may play a role on the preheating ahead of the burning zone, especially when the peak temperature is relatively higher (> 1000 °C). To fully understand the whole temperature profile and extinction behavior, further continuous study is recommended.

Ablation dynamics during laser interstitial thermal therapy for mesiotemporal epilepsy.

IntroductionThe recent emergence of laser interstitial thermal therapy (LITT) as a frontline surgical tool in the management of brain tumors and epilepsy is a result of advances in MRI thermal imaging. A limitation to further improving LITT is the diversity of brain tissue thermoablative properties, which hinders our ability to predict LITT treatment-related effects. Utilizing the mesiotemporal lobe as a consistent anatomic model system, the goal of this study was to use intraoperative thermal damage estimate (TDE) maps to study short- and long-term effects of LITT and to identify preoperative variables that could be helpful in predicting tissue responses to thermal energy.MethodsFor 30 patients with mesiotemporal epilepsy treated with LITT at a single institution, intraoperative TDE maps and pre-, intra- and post-operative MRIs were co-registered in a common reference space using a deformable atlas. The spatial overlap of TDE maps with manually-traced immediate (post-ablation) and delayed (6-month) ablation zones was measured using the dice similarity coefficient (DSC). Then, motivated by simple heat-transfer models, ablation dynamics were quantified at amygdala and hippocampal head from TDE pixel time series fit by first order linear dynamics, permitting analysis of the thermal time constant (τ). The relationships of these measures to 16 independent variables derived from patient demographics, mesiotemporal anatomy, preoperative imaging characteristics and the surgical procedure were examined.ResultsTDE maps closely overlapped immediate ablation borders but were significantly larger than the ablation cavities seen on delayed imaging, particularly at the amygdala and hippocampal head. The TDEs more accurately predicted delayed LITT effects in patients with smaller perihippocampal CSF spaces. Analyses of ablation dynamics from intraoperative TDE videos showed variable patterns of lesion progression after laser activation. Ablations tended to be slower for targets with increased preoperative T2 MRI signal and in close proximity to large, surrounding CSF spaces. In addition, greater laser energy was required to ablate mesial versus lateral mesiotemporal structures, an effect associated with laser trajectory and target contrast-enhanced T1 MRI signal.ConclusionsPatient-specific variations in mesiotemporal anatomy and pathology may influence the thermal coagulation of these tissues. We speculate that by incorporating demographic and imaging data into predictive models we may eventually enhance the accuracy and precision with which LITT is delivered, improving outcomes and accelerating adoption of this novel tool.

Analysis of simplified heat transfer models for thermal property determination of nano-film by TDTR method

Heat transfer in nanostructures is of critical importance for a wide range of applications such as functional materials and thermal management of electronics. Time-domain thermoreflectance (TDTR) has been proved to be a reliable measurement technique for the thermal property determinations of nanoscale structures. However, it is difficult to determine more than three thermal properties at the same time. Heat transfer model simplifications can reduce the fitting variables and provide an alternative way for thermal property determination. In this paper, two simplified models are investigated and analyzed by the transform matrix method and simulations. TDTR measurements are performed on Al–SiO2–Si samples with different SiO2 thickness. Both theoretical and experimental results show that the simplified tri-layer model (STM) is reliable and suitable for thin film samples with a wide range of thickness. Furthermore, the STM can also extract the intrinsic thermal conductivity and interfacial thermal resistance from serial samples with different thickness.

Application of Finite Difference Method for determining lunar regolith diurnal temperature distribution

This study was performed in order to verify viability of using finite difference method and proposed simple astrometrical model for modelling heat transfer in lunar regolith. The concept was examined by developing FD model of heat flow for upper 0,9 m of lunar regolith, and comparing obtained results with in situ measurements provided by Apollo 15 and 17 heat flow experiments. The model was based on FDM approximation of Fourier’s law for one dimensional transient heat flow. Both constant and temperature-dependent thermophysical properties of lunar regolith were obtained from in situ measurements. Thermal boundary conditions were assumed on in situ measurements and on remote sensing based analytical model. In order to approximate Sun's position at lunar sky, simple analytical astrometric model of lunar rotation was developed. Matlab 2012a was used to conduct the calculations. Stable solutions were obtained for latitudes between 0 and 80°. Satisfactory agreement between Apollo 15 and 17 in situ measurements and FDM modelling was observed. A conclusion was reached, that both FDM and proposed astrometrical model are to be successfully applied for modelling heat transfer in lunar regolith.

One-dimensional model for axial thermal error in a micro high-speed spindle

This article proposes a robust and accurate axial thermal error model for a micro high-speed spindle. With measured temperatures, an inverse method is applied to obtain the heat source and temperature field in the spindle for demonstrating that there exists a uniform temperature distribution along the axial direction within the motor range. Hence, a simple one-dimensional heat transfer model is established to acquire the temperature and resulting thermal errors using only one measured temperature on housing surface. Jumped displacement when the spindle starts and stops and the nonlinear deformation on the bearings are satisfactorily treated in the model. The results show that the estimated thermal errors agree with the measured data for both constant and various speeds. In addition, the results reveal that spindle speed significantly affects the maximum thermal error. A short processing time is an advantage of the proposed method. The model promises effective integration in machine tools for compensating thermal errors.

Dimension- and shape-dependent thermal transport in nano-patterned thin films investigated by scanning thermal microscopy

Scanning thermal microscopy (SThM) is a technique which is often used for the measurement of the thermal conductivity of materials at the nanometre scale. The impact of nano-scale feature size and shape on apparent thermal conductivity, as measured using SThM, has been investigated. To achieve this, our recently developed topography-free samples with 200 and 400 nm wide gold wires (50 nm thick) of length of 400–2500 nm were fabricated and their thermal resistance measured and analysed. This data was used in the development and validation of a rigorous but simple heat transfer model that describes a nanoscopic contact to an object with finite shape and size. This model, in combination with a recently proposed thermal resistance network, was then used to calculate the SThM probe signal obtained by measuring these features. These calculated values closely matched the experimental results obtained from the topography-free sample. By using the model to analyse the dimensional dependence of thermal resistance, we demonstrate that feature size and shape has a significant impact on measured thermal properties that can result in a misinterpretation of material thermal conductivity. In the case of a gold nanowire embedded within a silicon nitride matrix it is found that the apparent thermal conductivity of the wire appears to be depressed by a factor of twenty from the true value. These results clearly demonstrate the importance of knowing both probe-sample thermal interactions and feature dimensions as well as shape when using SThM to quantify material thermal properties. Finally, the new model is used to identify the heat flux sensitivity, as well as the effective contact size of the conventional SThM system used in this study.

Amperometric solid electrolyte NOx sensors – The effect of temperature and diffusion mechanisms

The diffusion mechanism of a zirconia-based amperometric NOx sensor was examined by studying the effect of sensor temperature on sensor output. The structure and the exact dimensions of a production automotive NOx sensor were first measured using x-ray tomography. A simplified heat transfer model was employed and validated to estimate the sensor temperature based on sensor heater power. The sensor temperature was then changed by changing the sensor heater power to evaluate different diffusion mechanisms. Normal multi-component diffusion mechanism, Knudsen diffusion mechanism and a combination of both mechanisms (Normal and Knudsen diffusion) were evaluated at different sensor temperatures. The normal multi-component diffusion mechanism was experimentally found to be the dominant diffusion mechanism that affects the diffusive flow through the sensor diffusion barriers. A sensor model was developed based on this dominant diffusion mechanism to predict NOx concentration. Finally, the sensor model output for NOx was validated with the experiments at different Diesel engine operating conditions with different species concentrations.

Crowd Simulation Incorporating Thermal Environments and Responsive Behaviors

AbstractCrowd simulation addresses algorithmic approaches to steering, navigation, perception, and behavioral models. Significant progress has been achieved in modeling interactions between agents and the environment to avoid collisions, exploit empirical local decision data, and plan efficient paths to goals. We address a relatively unexplored dimension of virtual human behavior: thermal perception, comfort, and appropriate behavioral responses. Thermal comfort is associated with the ambient environment, agent density factors, and interpersonal thermal feedback. A key feature of our approach is the temporal integration of both thermal exposure and occupant density to directly influence agent movements and behaviors (e.g., clothing changes) to increase thermal comfort. Empirical thermal comfort models are incorporated as a validation basis. Simple heat transfer models are used to model environment, agent, and interpersonal heat exchange. Our model’s generality makes it applicable to any existing crowd steering algorithm as it adds additional integrative terms to any cost function. Examples illustrate distinctive emergent behaviors such as balancing agent density with thermal comfort, hysteresis in responding to localized or brief thermal events, and discomfort and likely injury produced by extreme packing densities.

Calculation of the Intensity of Physical Time Fluctuations Using the Standard Solar Model and its Comparison with the Results of Experimental Measurements

The article reviews the possibility of describing physical time as a random Poisson process. An equation allowing the intensity of physical time fluctuations to be calculated depending on the entropy production density within irreversible natural processes has been proposed. Based on the standard solar model the work calculates the entropy production density inside the Sun and the dependence of the intensity of physical time fluctuations on the distance to the centre of the Sun. A free model parameter has been established, and the method of its evaluation has been suggested. The calculations of the entropy production density inside the Sun showed that it differs by 2-3 orders of magnitude in different parts of the Sun. The intensity of physical time fluctuations on the Earth's surface depending on the entropy production density during the sunlight-to-Earth's thermal radiation conversion has been theoretically predicted. A method of evaluation of the Kullback's measure of voltage fluctuations in small amounts of electrolyte has been proposed. Using a simple model of the Earth's surface heat transfer to the upper atmosphere, the effective Earth's thermal radiation temperature has been determined. A comparison between the theoretical values of the Kullback's measure derived from the fluctuating physical time model and the experimentally measured values of this measure for two independent electrolytic cells showed a good qualitative and quantitative concurrence of predictions of both theoretical model and experimental data.

Note on the Rate and Energy Efficiency Limits for Additive Manufacturing

SummaryWe review the process rates and energy intensities of various additive processing technologies and focus on recent progress in improving these metrics for laser powder bed fusion processing of metals, and filament and pellet extrusion processing of polymers and composites. Over the last decade, observed progress in raw build rates has been quite substantial, with laser metal processes improving by about 1 order of magnitude, and polymer extrusion processes by more than 2 orders of magnitude. We develop simple heat transfer models that explain these improvements, point to other possible strategies for improvement, and highlight rate limits. We observe a pattern in laser metal technologies that mimics the development of machine tools; an efficiency plateau, where faster rates require more power with no change in energy nor rate efficiency.

Design and Testing of a Micro Thermal Sensor for Non-Contact Surface Defect Detection

This paper presents an experimental study on a new concept of a surface defect detection method, in which surface defects will be detected by monitoring a change in heat flow between a micro thermal sensor and a smoothly-finished measuring surface such as magnetic disks, sapphire substrates and so on. In the proposed method, the micro thermal sensor is designed to detect surface defects without any contacts in between them. Since the change in heat flow across the gap is utilized, the method is expected to find out both the convex and concave defects. Searching for the possibility of the non-contact surface defect detection by the micro thermal sensor, in this paper, a simple heat transfer model is established to estimate the change in heat flow due to the change in gap between the measuring surface and the sensor surface. Some basic experiments are also carried out by using prototype micro thermal sensors, each of which is composed of a pair of electrodes and a thin metal film resistor, fabricated on both the silicon and glass substrates.

Heat transfer analysis of cylindrical anaerobic reactors with different sizes: a heat transfer model.

The temperature is the essential factor that influences the efficiency of anaerobic reactors. During the operation of the anaerobic reactor, the fluctuations of ambient temperature can cause a change in the internal temperature of the reactor. Therefore, insulation and heating measures are often used to maintain anaerobic reactor's internal temperature. In this paper, a simplified heat transfer model was developed to study heat transfer between cylindrical anaerobic reactors and their surroundings. Three cylindrical reactors of different sizes were studied, and the internal relations between ambient temperature, thickness of insulation, and temperature fluctuations of the reactors were obtained at different reactor sizes. The model was calibrated by a sensitivity analysis, and the calibrated model was well able to predict reactor temperature. The Nash-Sutcliffe model efficiency coefficient was used to assess the predictive power of heat transfer models. The Nash coefficients of the three reactors were 0.76, 0.60, and 0.45, respectively. The model can provide reference for the thermal insulation design of cylindrical anaerobic reactors.

A revised correlation based on heat transfer model of slug flow in mini/micro-channels

As flow boiling in mini/micro-channel, slug flow was corresponding to the optimal heat transfer ability owing to the evaporation of thin liquid film. Based on the heat transfer mechanism of liquid film evaporation, a simplified heat transfer model of slug flow was proposed. Li-Wu heat transfer correlation (Int J Heat Mass Transf 53(9): 1778‑1787, 2010) was revised by introducing evaporation parameter, C e . With the evaporation parameter, C e , the revised correlation predicted the slug flow database with MAE (Mean Absolute Error) 25.14%, which improved the prediction accuracy remarkably.