Why Engineering Curricula Should Include Alternative Methodology for Heat Transfer Coefficient

The paper presents the results of a study of radiative-convective heat transfer at film boiling of a liquid on a vertical heated plate. Both a steady-state problem of heat transfer and a transient problem were considered. The latter describes the instantaneous (flash) boiling up of a liquid on a heated surface. The novelty of the present study is the use of the optically thick medium approximation in a mathematical model when studying the process of radiation-convective heat transfer in the film boiling regime. For the first time, radiation heat transfer was considered for an optically thick medium. An analytical solution of the steady-state problems is obtained for boundary conditions involving a constant wall temperature and a constant wall heat flux. The effect of radiation and the temperature difference between the wall and liquid on the temperature profiles in the vapor phase is shown. The effect of radiation becomes more pronounced with an increase in the temperature difference between the wall and the liquid. As a result of solving the transient problem, the variation in time of the temperature profile and the heat transfer coefficient in the vapor film were obtained. The effect of radiation (Stark number) on the heat transfer coefficients is elucidated. An increase in the radiative heat flux leads to an increase in the Nusselt number, as well as the time it takes for the heat transfer process to reach a steady-state regime.

The efficient use of energy is a crucial goal related to the world economy, and efforts are aimed at reducing energy demand for the same quality and quantity of products and services. The drive to improve energy efficiency in the context of technological development, as well as the intensification of industrial and other processes, imposes a constant demand for more efficient heat exchange devices and systems. As devices in which the heat transfer process is crucial, heat exchangers play an essential role in many engineering and other applications. There are several generic techniques for improving the performance of thermal devices, which ultimately manifest through an increase in the heat transfer coefficient as the primary efficiency indicator. These methods include initiating or intensifying the turbulent flow of working fluids, increasing the heat exchange surface by applying specific shapes and additional elements, and using working fluids with improved thermophysical characteristics. This work presents a model for optimising a heat-exchanging surface with cylindrical fins based on the assumption that the material has a constant volume. In doing so, the maximum heat flux at the optimal diameter of the cylindrical fins made of different materials is compared. This research was conducted through a combination of three methods, namely, analytical, numerical (based on the computational fluid dynamic (CFD) technique), and experimental support, for the verification of the obtained results. The optimisation model is based on the analytical and numerical simulation of the heat flux through the fins to obtain the relevant thermophysical parameters of the investigated fin profiles. The optimisation of cylindrical fin profiles is performed for three different fin materials (steel, aluminium, and copper) based on the constant heat transfer coefficient and for different fin materials based on variable heat flux. The heat flux change along the fin was evaluated using analytical and numerical methods. The analysis showed that, due to the trade-off between the convective heat transfer surface area for each of the fin materials, an optimal fin diameter could be identified. Furthermore, the temperature along the cylindrical fin was evaluated through analytical, numerical, and experimental methods. The numerical simulation of the fin model was performed using CFD simulations, and the practical experimental research was carried out on a physical installation using a TESTO 875-2 thermal camera to obtain a clearer picture of the temperature profile. The results include heat flux through cylindrical fins as a function of the fin diameter, the material used, and the convective heat transfer coefficient. Moreover, a comparison of analytical and numerical solutions for the heat flux of cylindrical fins is presented. The temperature profile as a function of the fin element height x, obtained using three methods, has been used as a possible comparison between the methods. The optimisation of the fins has implications on the heat transfer efficiency, as well as on the material used to build the thermal devices and other heat-exchanging equipment.

Computation of airflow effects on heat and mass transfer in a microwave oven

This study investigates the thermal performance of a passive vertical aluminum heat sink with plate fins through combined experimental measurements and numerical simulations. Using a custom-made experimental apparatus which used water as the heat source, heat transfer rate was determined, and heat transfer coefficient was compared against established empirical correlations, demonstrating good agreement. A 3D steady-state mathematical model was developed to capture the conjugate heat transfer problem of conduction and natural convection, with buoyancy-driven airflow modeled with the incompressible ideal gas law. The problem was solved numerically using the finite volume method through ANSYS Fluent 18.2 solver and validated against experimental data and analytical correlations, exhibiting good agreement throughout. Parametric analysis followed, investigating the influence of various base (50, 65, 80 °C) and ambient (19, 24, 29 °C) temperatures, resulting in base-to-ambient temperature differences from 21 to 61 °C. Increasing this temperature difference led to a significant increase in heat transfer rate, while heat transfer coefficient increased and overall thermal resistance decreased moderately. Additionally, a Nusselt–Rayleigh (Nu–Ra) number correlation, consistent with ranges reported in the literature, was derived, providing the scaling to predict the thermal performance of similar natural convection-governed heat sinks. The validated computational methodology, combined with obtained experimental and numerical results, presents a foundation for future studies focused on more complex heat sink geometries and physics.

This paper presents an experimental investigation on the co-current downward condensation of R134a inside a tube-in-tube heat exchanger. The test section is a 0.5 m long double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is constructed from smooth copper tubing of 9.52 mm outer diameter and 8.1 mm inner diameter. The condensing temperatures are between 40 and 50°C, heat fluxes are between 9.78 and 50.69 kW m−2. The temperature difference between the saturation temperature of refrigerant and inlet wall varies between 1.66–8.94°C. Condensation experiments are done at mass fluxes varying between 340 and 456 kg m−2s−1 while the average qualities are between 0.76–0.96. The quality of the refrigerant in the test section is calculated considering the temperature and pressure measured from the test section. The pressure drop across the test section is directly measured by a differential pressure transducer. The average experimental heat transfer coefficient of the refrigerant is calculated by applying an energy balance based on the energy transferred from the test section. Experimental data of annular flow are examined such as the alteration of condensation heat transfer coefficient with the vapor average quality and temperature difference respectively according to different mass fluxes and condensing temperatures. The relation between the heat flux and temperature difference, besides this, the relation between the condensation heat transfer coefficient and condensing pressure are shown comparatively and the effects of mass flux and condensation temperature on the pressure drop are also discussed. The efficiency of the condenser is considered comparing with various experimental data according to tested condensing temperatures and mass fluxes of refrigerant. Some well known correlations and models of heat transfer coefficient were compared to show that annular flow models were independent of tube orientation provided that annular flow regime exists along the tube length and capable of predicting condensation heat transfer coefficient in the test tube.

Fouling is a common, fundamental and costly problem in heat transfer systems, which reduces thermal efficiency of equipment, increases the energy loss and causes strong damage to the heat transfer equipment in various industries. The main causes of fouling on the heat transfer surfaces are salts with inverse temperature-solubility in the fluid which calcium sulfate is one of the most important of them. In this paper, the effect of calcium sulfate fouling on the heat transfer coefficient in subcooled flow boiling was investigated. The fouling mass of calcium sulfate on the heat transfer surface was also calculated. In the experiments carried out in this study, flow rate (2.5–11.5 l/min), solution concentration (1.75–2.2 g/l), bulk fluid temperature (55–75 ℃), and heat flux (8-95 kW/m2) were variables at the mentioned ranges. The results showed that the maximum deviation in the uncertainty analysis was related to the difference between the inlet and outlet temperature of the fluid, followed by the temperature difference between the wall temperature and the bulk fluid temperature. Also, the analysis of the experimental data revealed that increasing the salt concentration, the bulk temperature, and the heat flux of the solution, the mass of deposited calcium sulfate on the heat transfer surface increases with time, resulting in a decrease in the heat transfer coefficient. Careful analysis of the experimental data also showed that the solution concentration has more important role than the heat flux and the fluid bulk temperature in fouling formation.

Comparison of heat transfer in liquid and slush nitrogen by numerical simulation of cooling rates for French straws used for sperm cryopreservation

Heat transfer enhancement in natural convection between vertical and downward inclined wall and air by pulsating jets

Investigations on heat and mass transfer characteristics of falling film horizontal tubular absorber

In the paper, the authors analyzed the solution of the differential equation of non-stationary heat conduction for an unbounded plate during the heat exchange of plate surfaces with the surrounding medium according to Newton’s law at a constant temperature of the medium. To use the results of solving the equations in the drying of thin flat materials, the dependence of the heat transfer coefficients on temperature and moisture content was studied. As a result of studying and analyzing a number of literature sources, the regularities of the change in the heat transfer coefficients during drying are established with high reliability. Studies of drying of thin wet plates of white and red clays with known heat transfer coefficients have shown that for small values of the heat transfer criterion of the Bio and small temperature gradients over the section of a thin material, application of the results of solutions of the heat transfer equations gives completely satisfactory agreement between the calculated and experimental values of the temperatures and the duration of drying. It is established that for small Bio numbers, the main factor is the external heat and mass transfer of the surface of the material with the surrounding medium and the rate of drying depends little on internal mass transfer. It is shown that the use of numerical methods for solving differential equations is possible with varying degrees of approximation only for accurate and reliable dependences of heat and mass transfer coefficients on moisture content and temperature. For a number of materials with known heat transfer coefficients, the use of analytical methods in calculations is of considerable interest and brings the theory closer to the practice of drying.

A heat sink is a device used to dissipate heat generated by electrical and electronic devices; it is classified as an active or passive heat sink according to the cooling process. The present study aims to evaluate the effect of forced vibration on heat sink thermal performance. For this purpose, a heat sink with rectangular longitudinal fins with regular aluminum sections has been selected. The heat sink has nine fins, each measuring 100 × 130 mm and having a thickness of 2 mm. These fins are firmly attached to a 100 × 100 × 10 mm aluminum base that contains a 140 W electric heater. It was installed at the middle height of a 75 × 14× 12 cm vertical duct. It was connected to a system that generated vibrations and the electrical power supply. A test rig contains various measurement devices, including a temperature data logger, an anemometer, and voltage and current meters. The tests were done with three different heat flux amounts: 150, 230, and 360 W/m2. The frequency range of the vibrations was 0–50 Hz, and the amplitude ranged from 0.07 to 6.99 mm using free convection. The analysis showed a linear relationship between the heat transfer coefficient and the modified Rayleigh number. This was at a frequency of 0 Hz and a heat flux ranging from 5 to 12 W/m2. Throughout the free-convective heat transfer range, even though vibration increased the heat transfer coefficient, it was found that the maximum heat transfer coefficient for overall heat fluxes was reached at 50 Hz. Regarding 150, 230, and 360 W/m2, respectively, this coefficient increased by 160, 59.5, and 55.2% compared to 0 Hz. Moreover, vibration has contributed to increased air mass flow at 50 Hz (8.8, 12.7, and 25%) compared to 0 Hz. Also, the results showed that the fin's efficiency decreased due to vibration by 9.8, 9.8, and 11.8% for 150, 230, and 360 W/m2, respectively.

Mixed convection heat transfer of one-stage spherical particle layer packed in an open shallow rectangular cavity

The investigation of relation between radiative and convective heat transfer coefficients at the ceiling in a cooled ceiling room

The characteristics of convection boiling heat transfer coefficients and two-phase flow pressure drops were measured for R-134a and HFO-1234yf flowing in smooth tube with inner diameter of 3.9 mm. The range of mass flux is between 200 to 500 kg/m2s, heat flux ranging from 5.67 to 26.18 kW/m2, evaporating temperature ranging from 6℃ to 15℃. Results showed that, the heat transfer coefficients of R-134a and HFO-1234yf increase with mass flux, heat flux and saturation temperature. The effect of heat flux on the heat transfer was large at low vapor quality, the effect of mass flux on the heat transfer was large at high vapor quality, and the heat transfer coefficient of HFO-1234yf is almost same as that of R-134a. For the pressure drops, under the same experimental conditions, the pressure drops of R-134a are higher than that of HFO-1234yf. Between experimental data and correlations proposed for the heat transfer coefficients, Wattlet and Chato’s correlation can predict data in smooth tube within 50%. For the pressure drops, Friedel’s correlation can predict the data within 30%.

The unsteady state overall heat transfer coefficient in a chemical heat pump reactor: the [formula omitted]– [formula omitted] system