Numerical Heat Transfer Model Research Articles

Comparison of Numerical Models of Flow and Heat Transfer Through Porous Medium in a Vertical Channel

Porous medium modelling technique has opened up ways for number of numerical studies to investigate the performance of many devices that involve heat exchanging process. Such modelling technique not only avoids huge cost and time as compared to experimental analysis but also makes computationally less time-consuming as in case of numerical simulation by exact geometry modelling of porous materials. In this regard the present paper analyses two different thermal models namely local thermal equilibrium model and local thermal non equilibrium model along with two different flow models namely Darcy flow model and Darcy extended Forchheimer model. Suitability of the mentioned models in predicting heat transfer through metal foam and wire mesh porous medium is examined subjected to variations in structural aspects of the porous medium that could be primarily represented by variation in porosity and pore density. For this purpose, a vertical channel subjected to constant heat flux capable of housing porous medium reported in literature is numerically modelled and air flow is numerically simulated through the channel. A variety of structural configuration (combination of different porosity and pore density) of the mentioned porous media are considered and among the mentioned flow and thermal models, best suited models for predicting flow and heat transfer through such medium are identified with appropriate justifications. It is revealed from the present study that, Darcy-Forchheimer and LTNE models are best suited to predict flow and heat transfer through porous media than the basic Darcy and LTE models.

Numerical optimization of a multistage sorption compressor

Sorption compressors are driven by thermal cycles and have no moving parts, excluding some passive check valves. Such compressors are suitable for powering Joule-Thomson (JT) cryocoolers and can provide reliable and vibration free active cooling system with a potential for high reliability and long operating life. The thermal cycle consists of cooling and heating a sorbent material which is installed in a sorption cell, where the heating is obtained by an inner electric heater and cooling is obtained by the surrounding via the sorption cell envelope. The investigation and optimization of the sorption cells were conducted in previous work, at steady state conditions, by a one-dimensional heat and mass transfer numerical model. The current paper presents a dynamic numerical model of sorption compressors which consist of several sorption cells. The numerical model allows one to three compression stages, with any number of sorption cells at each stage. The model enables the investigation of dimensional parameters and operational parameters, and provides the low and high pressures, pressure fluctuations, and compressor’s efficiency. The current investigation focuses on a three-stage compressor for nitrogen, with low and high pressures of 0.2 and 8 MPa, respectively, and a mass flow rate of about 11 mg/s.

DEVELOPMENT OF VENTILATED CLOTHING DESIGN PRINCIPLES FOR PROTECTION AGAINST BIOLOGICAL HAZARDS

A set of ventilated clothing to protect against tick-borne infections has been proposed. In addition to the protective function, such a kit allows you to normalising the thermal state of a person at elevated temperatures or physical activity by increasing the intensity of heat and moisture removal from the human body. A numerical model of heat transfer in ventilated air layers was developed. With its help, studies of heat and mass transfer were carried out under conditions of maximum activity of blood-sucking insects in the ambient temperature range of 15–25 °C. The balance of the human body was assessed for the level of physical activity typical for representatives of social groups with the highest risk of contracting tick-borne infections. It has been established that an air gap with a thickness of hag = 20 mm in ventilated clothing will provide a standard thermal state of a person and effective protection against infection with tick-borne infections.

Characterizing plant root parameters with deep learning-based heat pulse method

Accurate quantification of root structure is essential for understanding plant ecological system. Nevertheless, the lack of in-situ root characterization methods has limited further research of the relevant field. Heat pulse (HP) technique has emerged as a promising, cost-effective in situ monitoring approach. The existing HP technique relies on the analytical solution of heat transfer model with the assumption that the medium under test is homogeneous. Thus, its performance is compromised in dealing with heterogeneous formations. The objective of this study is to establish a novel in-situ HP method to characterize root parameters based on a deep learning technique. With the fully connected neural networks (FCNN), the number of root fragments was firstly estimated by solving a classification problem, and then the size and location of each fragment were estimated by solving regression problems. All FCNNs were firstly pre-trained using synthetic dataset generated by the numerical heat transfer model. Although FCNNs showed promising accuracy on the synthetic testing dataset, they failed to provide satisfactory results on real-world experimental data due to the model error and observation noise, i.e., the deviations between the numerical model and actual situation. To this end, FCNNs were fine-tuned with some experimental data through transfer learning for performance improvement. Under controlled indoor experiment in a sandy soil, the proposed method provided satisfactory results for the estimation of root fragment number and diameters with 83.3% accuracy and RMSE of 1.87 × 10−4 m, respectively. The position estimation had relatively larger error, i.e., RMSE 2.41 × 10−3 m. This study is the first step in translating heat pulse signals to root parameters, which indicates the potential of combining HP technique and deep learning in studying root system.

Prediction of Freezing Time and Numerical Model of Heat Transfer during Freezing Process

Prediction of Freezing Time and Numerical Model of Heat Transfer during Freezing Process

Thermal performance assessment of cold chain chamber with vacuum insulation panel envelope layer

As one of the super thermal adiabatic materials nowadays, Vacuum Insulation Panel has slim thermal conductivity, which can be no more than 0.0040 W/(m·K) at an internal pressure of less than 10Pa. the excellent thermal performance makes it prosperous in cold chin experiments. Actually, Vacuum Insulation Panel is normally installed with other insulation materials as multilayer envelope in practice. However, influenced by the fluctuation of environmental factors and the difference of thermal & physical properties of materials, the overall thermal performance of multilayer insulation systems with Vacuum Insulation Panel different arrangement sequences is significantly various. A reasonable Vacuum Insulation Panel location plays an important role on the thermal performance. In this paper, the thermal insulation performance of multilayer under unsteady state condition was studied. Three chambers, with the same exterior size, that is, 300 × 300 × 300 mm3, were employed for the innovation purpose. The multi-layer insulation construction involves a 10 mm layer of Vacuum Insulation Panels and a 10 mm layer of polyurethane. And three different envelope structures, namely, internal insulation, sandwich insulation and external insulation were built according to the Vacuum Insulation Panel's location. The numerical heat transfer model and physical model were established. The simulative and experimental results were comprised and analyzed. The results implied that, to maintain the constant temperature in cold\hot chamber, the vacuum insulation panels are located on the face of temperature fluctuation side, i.e. the ambient side, and have a better overall insulation performance.

Heat and mass transfer in a metal hydride reactor: combining experiments and mathematical modelling

Heat transfer in porous metal hydride (MH) beds determines efficiency of MH devices. We present a COMSOL Multiphysics numerical model and experimental investigation of heat and mass transfer in a MH reactor filled with 4.69 kg of AB5 type alloy (Mm0.8La0.2Ni4.1Fe0.8Al0.1). To achieve an agreement between the model and experiments it is necessary to include a flow control device (inlet valve or flow regulator) into the model. We propose a simplified and easy-to-calculate boundary condition based on a porous domain with variable permeability at reactor inlet. The permeability of the domain is connected with hydrogen mass flow by a PID controller. Thus, boundary conditions for the inlet pressure and mass flow are coupled and heat transfer inside the reactor could be calculated without additional assumptions applied to heat and mass transfer in the MH bed.

Heat transfer analysis of single and double U-tube borehole heat exchanger with two independent circuits

Borehole heat exchanger (BHE) is a very suitable and cost-effective technology for heat storage and extraction of heat from the ground. In this paper, comprehensive analysis of major influencing parameters on thermal performance of single U-tube BHE (sBHE) and double U-tube BHE (dBHE) was performed by using validated numerical heat transfer model. Combined effect of increasing borehole size and soil or grout thermal conductivity (for different cases of shank spacing) on performance of sBHE and dBHE was studied. Detailed comparative analysis of thermal performance was made among sBHE and dBHE with small and large borehole sizes. The impact of borehole depth and diameter, fluid inlet temperature, mass flow rate, U-tube pipe material type and diameter on total heat transfer per unit borehole depth, borehole thermal resistance and thermal effectiveness was also investigated. The simulation result revealed that when the shank spacing is kept at high or low value in sBHE and dBHE (both with small and large borehole size), dBHE with larger borehole size has the highest thermal performance while sBHE with smaller borehole size has the lowest performance; and the highest and the lowest thermal resistance is obtained for sBHE with smaller borehole size and dBHE with larger borehole size, respectively. Grout with high thermal conductivity is preferred in terms of improving thermal effectiveness and reducing thermal resistance of the BHE when the soil thermal conductivity is sufficiently high; while low grout thermal conductivity is preferred with large borehole size and high shank spacing. For high soil thermal conductivity (4W/m.K), the heat transfer rate per unit borehole depth increases with grout thermal conductivity to a maximum value; and then further increasing of the grout conductivity results in smooth declining of the heat transfer in the BHE. Thus, ground and grout thermal conductivity have an optimal range to achieve a lower value of the BHE thermal resistance, and consequently for better BHE sizing. With increasing fluid inlet temperature (15oC to 42oC), the total heat transfer is improved by 51.5 W/m and 36.2 W/m for dBHE and sBHE respectively; and more heat is injected into the borehole when dBHE is used than sBHE, particularly at higher inlet temperature showing that dBHE is preferable than sBHE for system that works with higher fluid inlet temperature. The result obtained can be used as a quick reference for the design, operation optimization and performance study of ground coupled heat pump system where BHE is used as aborehole thermal energy storage.

Numerical investigation on fin configuration improvement of 2 K sub-atmospheric plate-fin heat exchangers for the superfluid helium cryogenic systems

• The corrugated-serrated fin was proposed for the first time as a new type of fin. • A new structural parameter Lw number was defined to better describe the fin. • Numerical model under low temperature conditions was established. • The performance of corrugated, serrated and corrugated-serrated fins was compared. In order to improve the flow and heat transfer performance of 2 K sub-atmospheric pressure plate-fin heat exchanger, a new type of corrugated-serrated fin is proposed by combining corrugated and serrated fin. The Lw number is defined as a new structural parameter to describe the new type of fin. The numerical model of flow and heat transfer in the fin channels takes into account the influence of low temperature conditions with variable physical properties and has been fully verified by experimental data. Numerical investigation is performed to study the thermohydraulic characteristics of serrated fin, corrugated fin and corrugated-serrated fin. Meanwhile, three performance evaluation criteria Colburn factor j , friction factor f and TE-factor are used to qualitative compare their flow, heat transfer and overall thermo-hydraulic performance. The results reveal that the heat transfer performance of new corrugated-serrated fins is the best. Colburn factor j of the new fins increases by 19.97%~38.22% compared with corrugated fin, and increases by 25.09%~33.83% compared with serrated fin. The flow performance of the corrugated-serrated fin is slightly worse, but the overall thermo-hydraulic performance is better, especially at low Reynold number. Besides, the effects of various structural parameters on the thermohydraulic characteristics of the corrugated-serrated is obtained. The conclusions provide theoretical guidance on the design of 2 K sub-atmospheric plate-fin heat exchanger.

Numerical study on instantaneous heat transfer characteristics of AC arc-fault

Studying the heat transfer characteristics of alternating current (AC) arc-fault to electrodes is a key issue in electrical fires. In this paper, an instantaneous heat transfer numerical model of AC arc-fault is developed based on the magneto-hydrodynamic principle. The temperature distribution of the AC arc at the microseconds level and the influence of heat transfer on electrodes at the seconds level when the arc heats are studied. The numerical simulation of the axial temperature of the electrodes is verified by experiments, and the temperature variation in the electrodes at different currents and times is discussed. The results show that the arc temperature varies periodically similar to the current at the microseconds level but it does not go out when the current passes zero. The high-temperature region of electrodes diffuses with the increase in current or time. However, the axial temperature gradient of the electrode decreases with time and increases with current. Furthermore, the range of temperature increase in the electrode position decreases with the increase in current and time, but the electrode position near the arc has a higher initial temperature increase.

Numerical investigation of turbocharger turbine temperature field based on conjugate heat transfer

Aiming at the issues of low accuracy and poor feasibility of the analytical results of the turbocharger turbine temperature field under operating conditions, a full-domain conjugate heat transfer numerical model was established by the conjugate heat transfer and finite volume method. The temperature field characteristics of each component of the turbocharger turbine were analyzed. The numerical and experimental test results were compared and analyzed. The global conjugate heat transfer model avoids the input of a large number of hypothetical data on the interface between fluid and solid in the traditional model, and makes the calculation process closer to the actual situation. Through the comparison with the experimental results, the accuracy of the turbine temperature field obtained by the global conjugate heat transfer model is more reasonable and more accurate than that of the traditional model, which verifies the reliability and accuracy of the global conjugate heat transfer model.

Analysis of the temperature influence on thermophysical properties in the three-dimensional numerical modeling of heat transfer in human biological tissue in the presence of a cancerous tumor

Analysis of the temperature influence on thermophysical properties in the three-dimensional numerical modeling of heat transfer in human biological tissue in the presence of a cancerous tumor

Numerical investigation of the effects of heterogeneous air gaps during high heat exposure for application in firefighter clothing

For accurate prediction of thermal protective performance of firefighter clothing, a realistic assumption about the heterogeneous distribution of air gaps underneath the clothing is necessary. In this study, a numerical model of heat transfer through realistic heterogeneous air gaps under flash fire exposure was developed. First, the models of heat transfer and fluid motion were validated with data from scientific literature. The verified model was further developed and then used in a subsequent parametric study to quantify effects of heterogeneous air gap distributions. The results revealed that the difference in terms of heat transfer and skin burn injuries between heterogeneous air gaps with contact folds and equivalent homogeneous air gaps was greater than that between heterogeneous air gaps with non-contact folds and equivalent homogeneous air gaps. Fold aspect ratios showed a more prominent impact on heat transfer and skin burn times in the case of contact folds compared to non-contact folds. Exposure times to skin burn were continuously prolonged with increasing air gap thickness from 6.4 to 19.1 mm for homogeneous air gaps and heterogeneous air gaps with non-contact folds, while for heterogeneous air gaps with contact folds, there was an optimum air gap thickness around 12.7–15.9 mm.

Parametric study and optimization of a full-scale converter transformer winding

Thermal and hydraulic performance of the converter transformer winding is essential for improving the reliability and economy of the oil-cooled converter transformer. In this paper, a full-scale axisymmetric numerical heat transfer model was developed for a unilateral valve winding of an oil-cooled converter transformer. This model was first validated using data from the reference. Effects of the oil inlet/outlet position and the block washer number on the oil velocity and disc temperature of the winding were then studied under different oil mass flow rates. The thermal, hydraulic and comprehensive performance of the winding was investigated using Colburn-j factor (j), Darcy friction factor (f) and JF factor (JF), respectively. Finally, the optimal oil inlet/outlet position and block washer number were identified and the performance of the winding in the optimized design was further evaluated. The results showed that both the oil inlet/outlet position and block washer number had a significant impact on the disc temperature of the winding. The number of overheated discs was reduced by 50% when the oil inlet was set near the outside wall of the winding, compared with the conventional design in which the oil inlet was set at the middle of the winding. Furthermore, the maximum and mean temperature rises of the winding were reduced and the comprehensive performance was improved by setting the block washers. The optimal design for the winding was that the oil inlet was allocated near the outside wall of the winding, the number of the block washer was 3 and the oil mass flow rate was 2mo. In this optimal design, the maximum and mean temperature rises of the winding were reduced by 56.1% and 63.9%, respectively, and the comprehensive performance was improved by 145%, compared with the conventional design.

Theoretical and numerical investigation of heat transfer characteristics of an integrated cookstove under different atmospheric pressures based on theoretical models of solid flame and impingement heat transfer

At present, the problems of the efficient use of energy and heat transfer characteristics of the stoves on plateaus have attracted much attention. In this work, a three-dimensional numerical calculation and theoretical analysis model of impact flame heat transfer was established for the integrated cookstoves widely used in China and verified with the experimental thermal efficiency value. It was found that, with decreased atmospheric pressure, the combustion intensity in the furnace decreases, the flame combustion temperature significantly decreases, the heat transfer rate of the stove decreases, and the total thermal efficiency decreases by 7.5% under pressure of 0.05 MPa compared with normal pressure. By using jet impinged and solid flame models, a heat transfer mathematical model suitable for the inner wok and water jacket of the integrated stove was established. The heat transfer error between theoretical calculation and three-dimensional numerical calculation was no more than 6%. In addition, some methods are proposed to improve the thermal efficiency of integrated stoves under a high-altitude and low-pressure environment. This study not only theoretically reveals the change law of heat transfer characteristics of convective shock and flame radiation heat transfer under low pressure but also has constructive theoretical guiding significance for the improvement of thermal efficiency of highland stoves.

Numerical Modelling of Heat Transfer in Fine Dispersive Slurry Flow

Slurry flows commonly appear in the transport of minerals from a mine to the processing site or from the deep ocean to the surface level. The process of heat transfer in solid–liquid flow is especially important for the long pipeline distance. The paper is focused on the numerical modelling and simulation of heat transfer in a fine dispersive slurry, which exhibits yield stress and damping of turbulence. The Bingham rheological model and the apparent viscosity concept were applied. The physical model was formulated and then the mathematical model, which constitutes conservative equations based on the time average approach for mass, momentum, and internal energy. The slurry flow in a pipeline is turbulent and fully developed hydrodynamically and thermally. The closure problem was solved by taking into account the Boussinesque hypothesis and a suitable turbulence model, which includes the influence of the yield shear stress on the wall damping function. The objective of the paper is to develop a new correlation of the Nusselt number for turbulent flow of fine dispersive slurry that exhibits yield stress and damping of turbulence. Simulations were performed for turbulent slurry flow, for solid volume concentrations 10%, 20%, 30%, and for water. The mathematical model for heat transfer of the carrier liquid flow has been validated. The study confirmed that the slurry velocity profiles are substantially different from those of the carrier liquid and have a significant effect on the heat transfer process. The highest rate of decrease in the Nusselt number is for low solid concentrations, while for C > 10% the decrease in the Nusselt number is gradual. A new correlation for the Nusselt number is proposed, which includes the Reynolds and Prandtl numbers, the dimensionless yield shear stress, and solid concentration. The new Nusselt number is in good agreement with the numerical predictions and the highest relative error was obtained for C = 10% and Nu = 44.3 and is equal to −12%. Results of the simulations are discussed. Conclusions and recommendations for further research are formulated.

Heat Transfer with Phase Change in a Multilayer Construction: Simulation versus Experiment

The latent heat storage in the layer of phase change material (PCM) exposed to dynamic changes in boundary temperature was investigated numerically and experimentally. The original numerical model of heat transfer with phase change using a mushy volume approach was proposed and validated. The main improvement in the proposed model in comparison to others is that the compaction of the mesh and longitude of the time step were chosen after analysis of its impact in the field of error. The model was tested in the case of thin layer structure of the triple glazing window with one cavity filled with phase change material paraffin RT18HC. The experimental validation was carried out in the climatic chamber under dynamic changes in external temperature (from 10 to 50 °C) in a daily cycle. The highest accuracy was obtained for space discretization of the control volume 1 mm thick (12 CV for 12 mm of PCM layer) and 5 min time step. The obtained RMSE values, although they cannot be directly compared because of the very different approaches to the simulations, show that the proposed algorithm is sufficiently accurate for the assessment of energy storage in the PCM window. Both the simulation and experiment proved that, under specific conditions, implementation of the PCM into the structure resulted in delaying the peak for around 4 h.

Fire resistance of 3D printed concrete composite wall panels exposed to various fire scenarios

PurposeFire safety of a building is becoming a prominent consideration due to the recent fire accidents and the consequences in terms of loss of life and property damage. ISO 834 standard fire test regulation and simulation cannot be applied to assess the fire performance of 3D printed concrete (3DPC) walls as the real fire time-temperature curves could be more severe, compared to standard fire curve, in terms of the maximum temperature and the time to reach that maximum temperature. Therefore, this paper aims to describe an investigation on the fire performance of 3DPC composite wall panels subjected to different fire scenarios.Design/methodology/approachThe fire performance of 3DPC wall was traced through developing an appropriate heat transfer numerical model. The validity of the developed numerical model was confirmed by comparing the time-temperature profiles with available fire test results of 3DPC walls. A detailed parametric study of 140 numerical models were, subsequently, conducted covering different 3DPC wall configurations (i.e. solid, cavity and rockwool infilled cavity), five varying densities and consideration of four fire curves (i.e. standard, hydrocarbon fire, rapid and prolong).Findings3DPC walls and Rockwool infilled cavity walls showed superior fire performance. Furthermore, the study indicates that the thermal responses of 3DPC walls exposed to rapid-fire is crucial compared to other fire scenarios.Research limitations/implicationsTo investigate the thermal behaviour, ABAQUS allows performing uncoupled and coupled thermal analysis. Coupled analysis is typically used to investigate combined mechanical-thermal behaviour. Since, considered 3DPC wall configurations are non-load bearing, uncouple heat transfer analysis was performed. Time-temperature variations can be obtained to study the thermal response of 3DPC walls.Originality/valueAt present, there is limited study to analyse the behaviour of 3DPC composite wall panels in real fire scenarios. Hence, this paper presents an investigation on the fire performance of 3DPC composite wall panels subjected to different fire scenarios. This research is the first attempt to extensively study the fire performance of non-load bearing 3DPC walls.

Comparison between typical numerical models for fluid flow and heat transfer through single rock fractures

Numerical models can explicitly characterize fluid flow and heat transfer processes in rock fractures including randomly distributed asperities and contacts and have been widely used to understand the behaviors of fluid flow and heat transfer in fractured rocks. The differences between three-dimensional and two-dimensional numerical models for single rock fractures were compared in terms of fluid flow and heat transfer behaviors and properties. Both the artificial fractures created by Brazilian tension method and the hypothetical fractures created by duplicating a fracture surface were considered. The results showed that hypothetical fractures and two-dimensional models cannot capture the channeling flow and the non-uniform thermal fronts in artificial fractures, but two-dimensional models can predict the similar streamlines and thermal fronts as hypothetical fractures. The relationship between heat transfer coefficient and flow rate in a single rock fracture can be described by a logarithmic function, and the heat transfer coefficients of two-dimensional and three-dimensional models can be well correlated by an empirical function of fracture surface areas. The comparison between three-dimensional and quasi-three-dimensional models of artificial fractures is also conducted. Both channeling flow and non-uniform thermal fronts from quasi-three-dimensional models became similar to the results of three-dimensional models with increasing numbers of computational elements.

Numerical analysis of heat transfer between air inside and outside the tunnel caused by piston action

Piston action can affect the thermal environment inside the tunnel by changing the air pressure and velocity. This is extremely unfavorable for tunnels in cold regions because it causes frost damage in cold season. Understanding the influence of piston action on the air temperature distribution inside the tunnel can help us determine the length and location of frost damage. In this paper, a heat transfer numerical model considering piston action is developed. And then, a high-speed railway tunnel in the northeast of China is taken as an example to explore the temperature distribution laws via computational fluid dynamic (CFD). Afterwards, the effects of air temperature outside the tunnel and train velocity on temperature distribution are analyzed. The results show that the piston action can accelerate the heat transfer between cold air outside and inside the tunnel, the anti-freezing length near tunnel entrance and exit must be extended based on the calculation result of existed empirical formula. Before the train enters the tunnel, the freezing length near tunnel entrance is 1158 m. After the train passing through the tunnel, the freezing length near tunnel entrance is between 1495 m and 1497 m when the air temperature outside the tunnel decreases from −1 °C to −15 °C. Under different train velocities (80 km/h, 130 km/h, 160 km/h, 200 km/h and 250 km/h), the freezing lengths near tunnel entrance are 1458 m, 1469 m, 1472 m, 1497 m, 1498 m and its near tunnel exit are 330 m, 342 m, 349 m, 356 m, 357 m, respectively. In the section of 50 m away from tunnel entrance, the air temperature near the train changes suddenly, while the air temperature far away from the train changes gently. After the train passing through the tunnel, the air temperature inside the tunnel is consistent to that outside the tunnel within a specific distance from entrance. This study provides a guideline and references for the structure design and the construction of anti-freezing measurements for the tunnels in cold regions.