Numerical Heat Transfer Model Research Articles

Annual thermal performance assessment of a regenerative evaporative cooling system under different climate conditions in Mexico

A numerical heat and mass transfer model with thermophysical properties dependent on the temperature, humidity ratio and atmospheric pressure was developed. The numerical model was verified and validated against literature, and it showed good agreements. The pseudo-transient model provides a low-cost computational tool to evaluate the potential and performance of a dew point evaporative cooling system for a wide range of extreme climate conditions (BWh, BSh, Cw and Aw from Köppen classification). A parametric analysis of different operational and design conditions in the evaporative cooler was conducted. Results show that there is an optimal channel length for given climatic conditions. The Dew-point evaporative cooling (DPEC) system showed that the best thermal performance corresponds to the climate very arid (Hermosillo – BWh) with 4018 comfort hours (83.1%) followed by the climate arid (Monterrey – BSh) with 3470 comfort hours (90.9%), the mild climate (Puebla – Cw) with 295 comfort hours (100%) and the warm climate (Cancun – Aw) with 3452 comfort hours (62.3%). Finally, an engineering correlation for constant atmospheric pressure and channel length was obtained ([Formula: see text] of 93%).

Numerical modelling of conjugate heat and mass transfer during hydrofluidisation food freezing in different water solutions

A novel method of hydrofluidisation food freezing is numerically investigated in this paper. This technique is based on freezing small food products in a liquid medium under highly turbulent flow conditions when the heat transfer coefficient is higher than 1 000 W⋅m−2⋅K−1, which depends on the operating and flow conditions. A numerical model was developed to characterise the freezing process in terms of the heat transfer and diffusion of liquid solution components into the food product. The study investigates the freezing process of spherical samples in binary solutions of ethanol (30%) and glycerol (40%) and ternary solution of ethanol and glucose (15%/25%). The developed model was employed to determine the concentration of the liquid solution in food samples and to quantify the effect of sample size, heat transfer coefficient, solution temperature and concentration on the process. The food sample size varied from 5 to 30 mm, and the heat transfer coefficients varied from 1 000 to 4 000 W⋅ m−2⋅ K−1. The results confirm that a freezing time of 15 min for 30 mm diameter samples or less than 1 min for 5 mm diameter samples can be achieved with the hydrofluidisation method. The solution uptake was influenced by the solution type, sample size and process parameters and varied from 8.9 to 35 g of solute per kg of product for ethanol-glucose and glycerol solutions, respectively. This paper quantifies the advantages and possible limitations of hydrofluidisation, which has not yet been entirely studied, especially in terms of the mass absorption of different solutes.

A review of modern numerical and analytical models of heat transfer in a dielectric layer during melting due to microwave radiation

In this work, numerical and analytical solutions of heat transfer in a dielectric layer during melting in the microwave field were considered. We considered solutions, where the source term was obtained based on the solution of Maxwell equation, as well as using the Lambert law. The conditions applicable for analytical solutions, allowing the parametric analysis, are determined. The areas of application of the technology of microwave melting of dielectrics, in particular with melting ice on water, defrosting products, etc., were also considered.

Numerical modeling of fluid flow and heat transfer in Kurşunlu geothermal field-KGF (Salihli, Manisa / Turkey)

Nowadays, the need for energy is increasing more and more. It is more difficult to acquire new resources in various fields than to preserve existing energy resources. Although Turkey is a very rich state in terms of various energy resources, misuse of these resources can even lead to conflicts that may occur between the states in forthcoming years. In today's economic conditions, we can only protect our energy resources with the correct way of management. In this context, it is very important to reveal the mechanisms that make up the geothermal systems, which are very common in Western Anatolia. In this study, how the fluid circulation mechanism in the geothermal system takes place, and under which conditions the infiltrating water is heated in the Kurşunlu geothermal field (KGF) have been examined. FEFLOW software was used in numerical modeling. Fluid flow and heat transfer equations are solved on a twodimensional vertical model using FEFLOW software. A variable-width finite element mesh consisting of 55,590 elements was created in this scope. Since triangular meshes are preferred in vertical models, the mesh produced according to the Delaunay method was used. All lateral boundaries are designed as a no-flow boundary condition. For boundary conditions, hydraulic heads on top of the model and temperature values at both the top and bottom of the model are defined. Additionally, initial values were produced for the entire Kurşunlu geothermal system under steady-state conditions, and a transient model was built to run for 700,000 days. The regional flow direction is towards to the North. The fluids are transmitted deeply and heated through fault zones and transported towards the surface. Convective flows start to form below -1000 m altitudes in the fault zones and in the geothermal aquifer widespread convective flows in deeper regions were formed, while smaller spread convective flows were formed near the surface and shallow depths of the aquifer. In the process of convective flow, heated fluids reach to Kurşunlu region and forms the spring. Finally, two more possible high-temperature areas have been identified, indicating that the flow vectors point to the surface.

Numerical modelling of radiative heat transfer in a polydispersion of ceramic particles under direct high-flux solar irradiation

The effects of polydispersity on radiative and interfacial convective heat transfer are investigated in particle–gas two-phase media for solar particle receiver applications. Non-grey radiative transfer is analysed using the collision-based Monte Carlo ray-tracing method. The Mie theory is employed to calculate radiative properties of particles. The finite volume method and the explicit Euler time integration scheme are used to solve the transient energy equations for the particle and gas phases. Three alternative approaches to modelling particle properties and thermal conditions are employed: (i) a novel discrete size model, in which particle groups within discrete size intervals are assigned individual properties and temperatures locally; (ii) a lumped size model, in which integral properties and a single temperature are assigned to the particle phase locally; and (iii) a monodisperse size model, in which properties are evaluated for the Sauter mean diameter of the polydispersion and a single temperature is assigned to the particle phase locally. Strongly size-dependent radiation absorption and interfacial convective heat transfer are predicted with the discrete size model for alumina particles. Particles smaller than 27.4μm located near the aperture absorb the solar irradiation and transfer heat to the gas phase most effectively. The angular spread of the incident solar radiation is found to have a negligible effect on the overall absorption, although the most uniform thermal conditions occur for the solar irradiation with the smallest confinement angle. The overall absorptance of alumina particles is higher by 3.4% and 2.7% than that of iron (III) oxide and mullite particles, respectively. The lumped and monodisperse size models allow for reduction of the computational time at the expense of lower accuracy and limited information about particle properties and thermal conditions.

Heat transfer enhancement in microchannels using ribs and secondary flows

PurposeThe purpose of this paper is to study the flow and heat transfer characteristics of microchannel heatsinks with ribs, cavities and secondary channels. The influence of length and width of the ribs on heat transfer enhancement, secondary flows, flow distribution and temperature distribution are examined at different Reynolds numbers. The effectiveness of each heatsink is evaluated using the performance factor.Design/methodology/approachA three-dimensional solid-fluid conjugate heat transfer numerical model is used to study the flow and heat transfer characteristics in microchannels. One symmetrical channel is adopted for the simulation to reduce the computational cost and time. Flow inside the channels is assumed to be single-phase and laminar. The governing equations are solved using finite volume method.FindingsThe numerical results are analyzed in terms of average Nusselt number ratio, average base temperature, friction factor ratio, pressure variation inside the channel, temperature distribution, velocity distribution inside the channel, mass flow rate distribution inside the secondary channels and performance factor of each microchannels. Results indicate that impact of rib width is higher in enhancing the heat transfer when compared with its length but with a penalty on the pressure drop. The combined effects of secondary channels, ribs and cavities helps to lower the temperature of the microchannel heat sink and enhances the heat transfer rate.Practical implicationsThe fabrication of microchannels are complex, but recent advancements in the additive manufacturing techniques makes the fabrication of the design considered in this numerical study feasible.Originality/valueThe proposed microchannel heatsink can be used in practical applications to reduce the thermal resistance, and it augments the heat transfer rate when compared with the baseline design.

Two-Dimensional Transient Heat Transfer Model for High-Temperature Laser-Scanning Confocal Microscopy

Abstract High-temperature laser-scanning confocal microscopy (HT-LSCM) has proven to be an excellent experimental technique through in situ observations of high temperature phase transformation to study kinetics and morphology using thin disk steel specimens. A 1.0 kW halogen lamp within the elliptical cavity of the HT-LSCM furnace radiates heat and imposes a nonlinear temperature profile across the radius of the steel sample. When exposed at the solid/liquid interface, this local temperature profile determines the kinetics of solidification and phase transformation morphology. A two-dimensional numerical heat transfer model for both isothermal and transient conditions is developed for a concentrically solidifying sample. The model can accommodate solid/liquid interface velocity as an input parameter under concentric solidification with cooling rates up to 100 K/min. The model is validated against a commercial finite element analysis software package, strand7, and optimized with experimental data obtained under near-to equilibrium conditions. The validated model can then be used to define the temperature landscape under transient heat transfer conditions.

Analysis of Heat Transfer through a Protective Clothing Package

Abstract The protective clothing packages, which protect the human body against hot factors in a foundry are in continuous development to increase their resistance and comfort of use. The problem of heat transfer through textiles is the active field of research and reliable numerical modeling of this process can be helpful to design high-quality protective products. Therefore, the numerical model of heat transfer through the package based on the aluminized basalt fabric was developed. The macroscopic geometry of weft and warp threads was reproduced in agreement with samples of plain weave basalt fabric. Mapping the stochastically distributed individual monofilaments in basalt threads, as well as modeling the heat transfer between them, was impossible at the microscopic level. Therefore, the weft and warp threads were modeled as a porous material with a homogeneous distribution of basalt and air in their structure. Data from measurements of the bare and aluminized basalt fabrics by the Alambeta device were used to determine the model parameters. The model was used to simulate the heat transfer through the protective package composed of the aluminized basalt fabric, wool clothing, and cotton underwear. A good agreement of model results was found for measurement results in such a package. The presented procedure allowed for the determination of the main thermal properties of tested basalt fabrics.

Heat extraction study of a novel hydrothermal open-loop geothermal system in a multi-lateral horizontal well

The efficient development of hydrothermal resource is an important focus in the global geothermal industry. A novel hydrothermal open-loop geothermal system in a multi-lateral horizontal well is presented to achieve the simultaneous exploitation of multi-layer reservoirs and accelerate the large-scale development of the hydrothermal resource. To investigate the detailed performance of the novel geothermal system, a novel 3D fluid flow and heat transfer numerical model is established based on the local thermal non-equilibrium and dual porosity theories. The flow and temperature fields are analyzed, while the pressure and temperature interferences between the lateral wellbores of the multi-lateral horizontal well system are comprehensively investigated. The influences of the injection flow rate, lateral-wellbore number, spacing between the injection segment and the main wellbore, fracture aperture on the heat extraction are studied. Furthermore, the thermal characteristics of the novel geothermal system and traditional doublet well system are compared for the first time. The investigation indicates that the flow impedance of the novel geothermal system can decrease by 75.38%, and its energy efficiency reaches 8 times that of the doublet well system. The findings can provide a key reference for scientific research and application of the novel and traditional geothermal systems.

Experimental and Numerical Modeling of Heat Transfer to Graphite Surface in Underexpanded Dissociated-Nitrogen Jets

Experimental and Numerical Modeling of Heat Transfer to Graphite Surface in Underexpanded Dissociated-Nitrogen Jets

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.