Convective Heat Transfer Conditions Research Articles

Analysis and optimization of effect factors of refrigeration performance of portable micro-refrigerator

The objective of this study was to ascertain an efficient portable cold container, considering the constrained space, low airflow velocity, and uneven heat distribution of a micro-refrigerator, which lead to in insufficient heat dissipation and low refrigeration efficiency. A thermoelectric micro-refrigerator model with a volume of 9083.8 cm3 was established under forced convection heat transfer conditions. Through single-factor and response surface experiments, the impact of thermoelectric elements configuration on refrigeration efficiency was identified. These elements include the input current of the Thermo Electric Cooler (TEC), the derating factor (actual power) of the cooling fan, and the height of the heat sink fin.The results indicate that optimizing the refrigeration conditions significantly enhances the refrigeration efficiency of the micro-refrigerator. At an ambient temperature of 25℃, with a fan derating factor of 0.77 (corresponding to an actual power of 0.8218 W), a heat sink fin height of 59.15 mm, and a TEC input current of 5.09A, under stable conditions,the cold side temperature can reach −11.3 ℃. The hot side temperature is 38.2 ℃, consistent with the response surface experiment result. Numerical simulation under these conditions reveals a cold side temperature of −12.4 ℃ and a hot side temperature of 41.9 ℃, showing a 9.7 % difference from the experimental results. The simulation data further demonstrate that the vertical arrangement of the cooling fan and heat sink can achieve a cold side temperature of −12.9 ℃, effectively maintaining the hot side temperature below 40 ℃. The thermoelectric system investigated in this paper holds promising potential for applications in portable fresh-keeping and refrigeration.

Engineering applications development through OHAM in heat and mass transfer during stagnant flow of second-grade nanomaterial

This paper investigates the stagnation point flow of a second-grade nanomaterial, considering nanofluid effects via thermophoresis and Brownian motion. The study addresses convective heat and mass transfer conditions within the flow induced by a stretching cylinder. The transformed systems are solved using the optimal homotopy solutions (OHAM) method, revealing the impact of various parameters on the quantities of interest. The results indicate that an increase in curvature, viscoelasticity, velocity ratio and injection parameters leads to an enhancement in velocity, while the opposite trend is observed due to suction. The viscoelasticity and curvature parameters also increase skin friction. Additionally, thermophoresis enhances the temperature and concentration fields, while Brownian motion reduces mass diffusion.

Research on the nonuniform contact characteristics and thermal behavior of friction pairs involving the radial conical buckling deformation in hydro-viscous drive

An equivalent model and a finite element method was established and used for simulating the change of contact state and pressure distribution of friction pairs under the axial pressure to obtain the radial nonuniform contact characteristics and temperature field of conical friction pairs during the soft-start condition. A two-dimensional temperature field calculation model was established based on the results of contact characteristics and convective heat transfer conditions, and the transient temperature field of friction pairs during soft-start was calculated by the finite difference method. Finally, the effects of the soft-start time, deformation degree, and thickness of the steel disk were studied. The results show that the temperature and contact pressure distribution of conical deformation friction pairs exhibit local high-temperature and high-pressure areas during the soft-start process. In the early stage of soft-start, the conical friction element locally contacts in its radial central area. The contact area spreads from the center to both sides as the soft start time increases, and two contact pressure peak areas are formed near the inner and outer diameters with the increase in axial pressure. The degree of axial deformation of conical steel disk has a significant impact on contact pressure. The larger the axial deformation, the pressure distribution would be more uneven. The distribution of temperature field is significantly affected by the contact pressure. It spreads from the middle to both sides, forming high-temperature regions in the pressure peak areas. Meanwhile, the temperature on the outer side of the friction element is higher than that of the inner side due to the influence of convective heat transfer. The duration of soft start also has a significant impact on the maximum temperature and maximum temperature difference. The results could provide a theoretical basis for exploring the correlation between the thermal buckling deformation and torque instability.

Design of Various Types of Fins On a Vertical Aluminium Plate Under Natural Convection Heat Transfer Condition For Experimental Investigation and Computational Fluid Dynamic Analysis

Heat transfer is a process by which internal energy from one substance transfers to another substance. Heat transfer, the movement of thermal energy from one place to another, can occur through three main mechanisms: conduction, convection, and radiation. Heat transfer also is important in our daily lives. In this paper, the study of various fins on a plate is done and a sample analysis of heat flow is done in order to predict the heat flow.

Study on the performance of thermoelectric refrigerator under natural convection heat transfer condition

Refrigeration using thermoelectric cooler (TEC) is a hotspot and frontier issue, but the forced heat dissipation apparatus poses problems that limit its application. In this paper, a thermoelectric refrigerator used under natural convection heat transfer conditions is presented, and numerical simulations and experimental analyses are performed. The maximum error between the simulation results and experimental results is 6.3%. The distributions of the temperature and velocity field in the refrigerator chamber are obtained, and the influences of the operating current and the height of fins on the temperature are analyzed. The novel design is that the number of thermoelectric legs in a single TEC is reduced to five pairs to reduce the heat generated on the hot side during operation. The performance of TEC is improved by increasing the average heat dissipation area. The results show that the air temperature in the chamber is finally stabilized at 12.2 ℃ under the ambient temperature of 29 ℃ and input electric power of 10.8 W. The air temperature difference inside and outside the thermoelectric refrigerator increases from 16.8 ℃ to 19.5 ℃ with the fin's height increasing, but the coefficient of performance (COP) changes little.

MATHEMATICAL MODELING OF COMFORTABLE CONDITIONS OF CONVECTIVE HEAT EXCHANGE OF HUMANS AND ENVIRONMENT

Estimating environmental characteristics and determining comfortable conditions is a rather complex task that requires powerful mathematical and computer equipment. The purpose of the article is to build a mathematical model and determine the comfortable conditions of convective heat transfer of man and the environment by temperature in the city of Mykolaiv. The article “Mathematical modeling of comfortable conditions of convective heat exchange of humans and the environment” presents experimental studies of the evaluation of the comfortable conditions of convective heat exchange of the human body and the surrounding environment based on mathematical calculations allow to clearly outline the comfort zones in the cold period of the year. The basis of the research is the mathematical modeling of the comfortable conditions of the human body, which is based on the use of the developed algorithm, the basis of which is the synthesis of two software products. The ability to import data made a significant impact on the synthesized work of the programs, which made it possible to transfer the calculations without loss of content without loss of time. Optimal solutions identified during the creation of the algorithm allowed to create a method of mathematical modeling of the comfortable conditions of the human body. The heat exchange between the human body and the temperature of the atmospheric air is estimated. The graphical dependence of the change of convective heat exchange on the atmospheric air temperature is constructed. An approximation of the curve on the basis of which the derivative of the function is determined. The levels of comfort of convective heat exchange of man with the environment are presented. The optimal temperature conditions for the human body in the cold period of the year are substantiated. The normal temperature threshold is 1.5-0.5 °C, where the human body is in a comfortable state and there is moderate heat transfer. For the middle level, the temperature is determined in the range of 2.0-1.49 and 0.49-0 °C, where the increase in heat transfer begins due to the increase in atmospheric air temperature, which leads to sweating. Lowering the temperature, in turn, leads to heat release from the body to warm up.

The coupled cooling effect of ventilation and spray in the deep-buried high-temperature tunnel

The utilization of ventilation and spray cooling technology is becoming increasingly popular in high-temperature workplaces due to its convenience and environmental friendliness. To investigate the coupled cooling effect of ventilation and spray in a construction tunnel, we established a droplet evaporation model under convective heat transfer conditions by combining the convective heat transfer theory and the droplet heat and mass transfer model. Based on this model, the study focused on the energy change of droplet evaporation in a moving medium by using numerical simulation in the context of a railroad construction tunnel. The material of the droplets used in the study was water. According to the simulation and field measurement results, it was obtained that, firstly, increased air volume of the tunnel and reduced airflow temperature could make the convective heat exchange stronger between the airflow and the wall. Secondly, the particle size of droplets formed by spraying was mainly 30–40 μm in the tunnel. A part of the droplets was evaporated in the high-temperature environment, and another part was trapped by the rock wall and ground. Third, adopting the coupled cooling method of ventilation and spray, the temperature and humidity fields in the tunnel changed significantly, and a low-temperature and high-humidity zone appeared near the nozzle. At the same time, the evaporation rate of droplets increased, the heat index of the workplace decreased, and the cooling effect was effective. Therefore, the coupled cooling method of ventilation and spray can effectively improve the thermal environment in high-temperature construction tunnels and enhance the thermal comfort of personnel. This study serves as a reference for the parameters and cooling effect of ventilation and spray cooling systems in construction sites.

Performance Evaluation Based on Exergy Analysis Through Partially Filled Metal Foams in Forced Convection

Abstract The intention of this paper is to present the numerical analysis of thermal performance and exergy transfer through high porosity metal foams filled partially in a horizontal pipe. The heater is embedded in the pipe's circumference and is assigned with known heat input. To enhance heat transfer, aluminum metal foam of pore density 10 with porosity 0.95 is inserted adjacent to the pipe's inner wall. To determine the optimal thickness of metal foam for enhancing its performance thermodynamically, metal foams with five different thicknesses (10, 20, 40, 60, and 80 mm) are examined in this research for a fluid velocity ranging from 0.7 to 7 m/s under forced convection heat transfer condition. The Darcy– Brinkman–Forchheimer and local thermal nonequilibrium (LTNE) models are used for forecasting the flow features and heat transfer through the metal foams, respectively. The numerical methodology implemented in this research is confirmed by comparing the present outcomes with the experimental outcomes accessible in the literature and found a fairly good agreement between them. The thermal performance is assessed in terms of heat transfer enhancement ratio and performance factor, and the thermodynamic performance is evaluated based on exergy analysis. In the exergy analysis, the parameters like mean exergy-based Nusselt number (Nue), merit function (MF), and nondimensional exergy destruction (I*) are considered for the evaluation. The result shows a better performance from partially filled metal foams than from completely filled metal foams.

Analysis of step-heating thermography methods for defect depth prediction

In this study, five theoretical equations of specific characteristic peak times are proposed to predict the defect depth for step-heating thermography. These equations were derived for test samples with surfaces under adiabatic and convective heat transfer conditions, where, as demonstrated from experiments, the effect of convection on depth prediction can be significant for materials with low thermal conductivities. Two glass fiber reinforced plastic (GFRP) samples with flat-bottom holes at various depths were inspected by step-heating thermography, and the defect depths were predicted by these five methods. Based on the experimental results, the prediction accuracy and the performance of these depth prediction methods are discussed.

Experience of non-uniform heat source/sink on the flow of micropolar nanofluid fluid with heterogeneous and homogenous chemical reaction

Due to the varied applications in industries, chemical reaction has a significant role in the perfect shape and size of the product that is dependent upon the proper cooling of the system. Therefore, the existing analysis reveals the impression of non-uniform heat source/sink on the micropolar nanofluid through a nonlinear vertical stretching surface for the interaction of heterogeneous and homogeneous chemical reactions. Oxide nanoparticle embedding with the traditional water is considered by introducing operational properties such as thermal conductivity and viscosity. The free convective water-based nanofluid past through a permeable medium interacts with the transverse magnetic field and the thermal buoyancy enriches the present discussion. Further, the convective heat transfer condition is useful to enhance the thermal properties as well. Imposing suitable transformations the present problem turned into ordinary and solved by employing the bvp5c built-in MATLAB code. Further, the important results are; inclusion of particle concentration favors for the enhancement in the shear stress as well as the heat transfer rate in conjunction with the resistive forces such as magnetic and porosity but the impact is reversed in case of couple stress coefficient however, retardation in the fluid temperature occurs for the increasing space and temperature dependent heat source.

Experimental and Numerical Investigation of a Multifunctional CFRP towards Heat Convection under Aircraft Icing Conditions

A combined experimental and numerical approach for the analysis of convective heat transfer from a multifunctional flat plate specimen under aircraft icing conditions is presented. The experimental setup including a heat control and measurement system that is installed in a de-icing test bed. The ambient temperature (θa=[253,283]K), air velocity (va={0,15,30}ms), and angle of attack (α={10,30}∘) are varied, and their influence on heat transfer during local Joule heating is discussed. The numerical approach utilises the results to compute the convective heat transfer coefficients (HTC) based on Newton’s convective heat transfer condition. Results indicate that the numerical model represents the heat transfer behaviour with high accuracy. The HTC for free convection was found to hold h¯≈2.5Wm2K and h¯≈[10,40]Wm2K for forced convection conditions with minor scattering. The increase in HTC under forced convection conditions has a significant effect on the overall heat transfer behaviour, resulting in high temperature gradients within the material. The functional optimisation of multifunctional structures will benefit from including application related convection conditions, dealing with resulting temperature fields by structural design. It is expected that multifunctional structures for de-icing as well as for structural energy storage, morphing structures, or stiffness adaptive structures with similar material constituents will benefit from this recognition.

Dynamic and kinematic characteristics of unsteady motion of a water-in-oil emulsion droplet in collision with a solid heated wall under conditions of convective heat transfer

The paper presents the experimental results of spreading, fingering and corona-like splashing of water-in-oil emulsion droplets based on n-decane, isoparaffin oil and distilled water in the range of Weber numbers We = 100–900, wall temperatures Tw = 20–80 °C and in collision with a smooth solid surface. The spreading features and separate convective rolling structures in emulsion droplets are examined at the late stages of wetting. The effect of Tw on the viscous dissipation energy and the maximum spreading coefficient is established. The validity of the assumptions explaining the contradictory behavior of the maximum spreading diameter of emulsion droplets with increasing temperature is studied. The assumption of unstable behavior of the emulsion droplet rim during spreading is proposed. The maximum emulsion corona diameter and height decrease linearly with the Tw growth due to the longer corona lifetime. The maximum corona diameter increases following the power law with increasing the Brinkman number due to an increase in the liquid viscosity and a decrease in Tw. The results help to predict the corona's behavior by determining the relationship between the viscous dissipation of the spreading liquid and the energy transferred to the liquid from the wall due to molecular (thermal) conduction.

A Liquid Metal-Enhanced Wearable Thermoelectric Generator.

It is a key challenge to continuously power personal wearable health monitoring systems. This paper reports a novel liquid metal-enhanced wearable thermoelectric generator (LM-WTEG that directly converts body heat into electricity for powering the wearable sensor system. The gallium-based liquid metal alloys with room-temperature melting point (24~30 °C) and high latent heat density (about 500 MJ/m3) are used to design a new flexible finned heat sink, which not only absorbs the heat through the solid-liquid phase change of the LM and enhances the heat release to the ambient air due to its high thermal conduction. The LM finned is integrated with WTEG to present high biaxial flexibility, which could be tightly in contact with the skin. The LM-WTEG could achieve a super high output power density of 275 μW/cm2 for the simulated heat source (37 °C) with the natural convective heat transfer condition. The energy management unit, the multi-parameter sensors (including temperature, humidity, and accelerometer), and Bluetooth module with a total energy consumption of about 65 μW are designed, which are fully powered from LM-WTEG through harvesting body heat.

Evolution Behaviors and Reduction Mechanism of Curing Residual Stresses in GLARE Laminates under a Hot-Pressing Condition.

Nowadays, variable preparation, forming and processing methods of fiber metal laminates are constantly developing to meet the requirements of different application fields, hence the characteristics and evolution of residual stresses under different manufacturing conditions deserve more attention. In this work, the evolution behaviors of curing residual stresses in GLARE under a hot-pressing condition were studied, and the residual stress reduction mechanism was also explained. Results suggested the FE prediction models of the entire cure process, verified by the fiber Bragg grating (FBG) sensors, were more precise than the traditional elastic model. Moreover, the stress evolution during the cure process mainly occurred in the cooling stage, in which the different coefficient of thermal expansion (CTE) of aluminum and GFRP played a major role. Meanwhile, curing shrinkage stress in the GFRP layer during the holding stage at curing temperature obviously influenced the final stress level. The residual stresses in GFRP layers differed by 9.6 MPa under a hot-pressing and autoclave condition, in which the convection heat transfer condition played a major role as it caused lower thermal stress in the holding stage and a smaller temperature gradient in the cooling stage. Considering this, a lower cooling rate could be a feasible way to obtain GLARE with lower residual stress under a hot-pressing condition.

Исследование способов мгновенного замораживания

Strawberries are in demand in confectionery and food industries. Berries are glazed, used in decorating of different confectionery or as a separate tender dessert. The growing season is limited to the summer period, and the demand for berries remains all year round. Low-temperature canning is one of the ways to preserve quality characteristics during the storage of berries. A freezing process is used during storage to maintain quality characteristics. Freezing under conditions of convective heat transfer at a temperature of 18 degrees below zero centigrade does not fully ensure the preservation of the organoleptic properties of the product. For this reason it becomes necessary to conduct an analysis of the existing methods of freezing. The following freezing methods were chosen for the exploring: freezing under conditions of convective heat transfer at a temperature of 18 degrees below zero centigrade, flash freezing under conditions of convective heat transfer at a temperature of 35 degrees below zero centigrade and cryogenic freezing in a carbon dioxide environment using ultrasonic radiation. The conducted studies and their analysis made it possible to obtain the following result: cryogenic freezing provides higher organoleptic characteristics of strawberries, both in frozen and thawed state. This is explained by the size of the forming ice crystals. To limit the growth of crystals, it is proposed to institute ultrasonic vibrations into a laboratory cryogenic freezing machine during the phase transition. The ultrasonic generator puts out oscillations with following parameters: ν≈880 kHz; I ≤0.4 W / cm2.Ultrasonic vibrations affect the strawberries until the temperature in the center of the berries reaches minus 50C.The selected modes of cryogenic freezing in a carbon dioxide environment with intensification of the phase transition are rational, since the freezing time is reduced and the number of crystallization centers increases, ensuring uniform freezing of all moisture. The storage of cryogenically frozen strawberries also showed better preservation. Keywords: CRYOGENIC FREEZING, FLASH FREEZING, CARBON DIOXIDE, GARDEN STRAWBERRY, ORGANOLEPTIC ESTIMATION, PHASE TRANSITION, ULTRASONIC VIBRATION

ANSYS Simulation of the Thermomechanical Behavior of a Large-Sized Composite Mandrel with Consideration of Viscoelasticity

The article addresses the modeling of the process of manufacturing a large-sized shell, given the thermomechanical behavior and viscoelasticity of the composite mandrel. The results of the experimental identification of viscoelasticity parameters of the examined material are presented. A numerical algorithm for adapting the experimental data for the ANSYS Mechanical APDL finite element analysis package is proposed. A Prony series expansion of the relaxation kernel is used as a model for describing the material behavior. The effect of temperature on the rate of relaxation processes is taken into account through the application of a temperature-time analogy according to the Williams–Landel–Ferry formula. The selected model with the calculated parameters was implanted into the commercial package of ANSYS Mechanical APDL. Simulation of two process steps of manufacturing a large-sized product was performed: winding and heat treatment of the shell. For this purpose, the quasistatic problem of mechanics and unsteady thermal conduction under conditions of convective heat transfer were solved by the finite element method. The influence of thermomechanical behavior of the mandrel material on the normal pressure acting on the mandrel surface as a function of temperature and force factors was estimated quantitatively and qualitatively. It was found that with respect to the nonlinear behavior of the composite material, the pressure level decreases by 50% compared to the case of using models of elastic behavior. This result justifies the importance of using complex models of material behavior in studying long-term technological processes, especially those associated with high-temperature effects.

Optimization of Parameters Affecting Anti-Icing Performance on Wing Leading Edge of Aircraft

In this article, one of the hazardous trouble in-flight situations called ice accumulation on wing leading edge of aircraft has been numerically investigated. While the target surface is kept constant temperature at Tw=263.15 K, the inlet temperature is taken constant at Tin=473.15 K and it enters to the system with m ̇=0.004 kg/s. The investigation starts with validation of numerical study utilizing airfoil type of NACA 0015 using constant piccolo tube dimensions, jet angle, distance among jets and distance between jet-to-target positions. The second and third stages are to analyze the anti-icing performance of changing positions of piccolo tube on X (4≤H/d≤8) and Y (-1.25≤L/d≤1.25) directions under different jet angles (30°≤≤150°). The fourth stage is to determine the optimum H/d, L/d, and ratios to increase anti-icing performance with maximum convective heat transfer and minimum pressure drop condition. The optimization study has been done using Response Surface Methodology (RSM). Finally, while the best anti-icing performance proposed design is achieved using° L/d=0.0, and H/d=4.0, the optimization results show that the L/d, and H/d values should be 55.45°, 0.0, and 4.0, respectively to achieve maximum heat transfer rate with minimum pressure drop value.

Нестаціонарний теплообмін у вертикальному циліндричному об’ємі, заповненому рідиною

The paper analyzes the conditions of convective heat transfer in the "limited volume" and in the "large volume". It is established that the process of heat exchange in the elements of the basic experimental setup in the system of experimental calculation method corresponds to heat exchange in a "large volume" under conditions of free convection. An experimental stand for the study of nonstationary heat exchange in the system "environment I ― body II", the main elements of which are two working cavities — the outer filled with water, volume V1, and the inner filled with the studied liquid medium volume V2, and V1 is larger V2 3 times. The results of experimental determination of heat transfer between the inner surface of a thin metal cylinder and the investigated liquid medium in a limited space for the system “environment I — body II” under the conditions of laminar regime of both the environment and the investigated liquid medium are presented. The heat transfer coefficient was investigated using the calculation-experimental method during heating and cooling of refined sunflower oil, distilled glycerin, sugar solution with a concentration of 50 %, 60 % under conditions of free convection. The experimental results are compared with the results of studies by well-known authors for "large volume" conditions. The conditions of the heat exchange process during the experiment at different directions of heat exchange in a "limited volume" (inside a thin-walled metal cylinder with the liquid under study) are described. It is substantiated that at the surface of a cylindrical metal wall in the process of heat exchange a thermal boundary layer is formed, within which the coolant temperature changes and heat transfer within the boundary layer occurs due to thermal conductivity. The criterion equations for determining the heat transfer coefficient during heating and cooling of the investigated liquid medium under free convection conditions are obtained.

Dynamic and Kinematic Characteristics of Unsteady Motion of a Water-in-Oil Emulsion Droplet in Collision with a Solid Heated Wall Under Conditions of Convective Heat Transfer

Dynamic and Kinematic Characteristics of Unsteady Motion of a Water-in-Oil Emulsion Droplet in Collision with a Solid Heated Wall Under Conditions of Convective Heat Transfer

Turbulence model assessment and heat transfer phenomena inside a rectangular channel under forced and mixed convection

A very high-temperature gas-cooled reactor (VHTR) is an attractive option for hydrogen production due to its high operating temperature. The reactor cavity cooling system (RCCS), a passive safety system of the VHTR, surrounds the reactor vessel with rectangular riser channels and removes decay heat from it. Under emergency operation conditions such as a decrease in the flow rate of the RCCS, the flow condition inside the RCCS riser can become a mixed convection condition accompanying heat transfer deterioration. To design and operate the VHTR with reliable safety, accurate predictions for various convective heat transfer conditions in RCCS riser channels are required. In this study, by comparing the results of an air flow visualization experiment, the predictability of the turbulence models was assessed for the convective heat transfer and complex flow characteristics near the corner region of the rectangular duct confirmed from the experimental study. In this paper, the heat removal rate through the test section of the visualization experiment was quantified under various convective heat transfer conditions, and local temperature measurement data from the experiment were presented to describe the heat transfer deterioration phenomena inside a rectangular riser in turbulent mixed convection. Linear and nonlinear eddy viscosity models were evaluated by comparing the heat transfer coefficient, local temperature, local flow structure including secondary flow, and turbulence quantities obtained from the experiment. Furthermore, improvements to turbulence models for enhancing the prediction of mixed convection heat transfer in rectangular riser ducts were proposed.