Forced Convection Conditions Research Articles

Estimation of forced heat convection in a rectangular channel with curved-winglet vortex generator: A machine learning approach

• It was determined that angle of curved winglet, plate temperature, and Re on heat transfer are important parameters. • The findings achieved from the experimental datas have been evaluated as a result of the comparison with the target value within the ±4% diversion interval for all Nu. • The designed artificial neural network ( ANN ) structure indicated optimum mean square error ( MSE ) of 1.4x10 -2 , average relative deviation ( ARD% ) of 0.1771 and correlation coefficient ( R 2 ) of 0.9879 in architecture of average Nu datasets. There are a limited number of studies in the literature on the effect of curved fins on heat transfer. In this study, the effect of novel fin geometry and angle of attack of winglet ( α =30 o , 60 o , and 90 o ) on heat convection is estimated using a machine learning method. Airflow in the rectangular channel is investigated under constant heat flux ( q ''=100 W/m 2 ) and turbulence regime (5683≤Re≤17049) by experimental studies. Improvements in heat transfer are observed at different temperature values (T=30 o C, 50 o C and 70 o C) of the plate on which the blades were attached. In order to investigate the effect of input parameters on the prediction accuracy, an artificial neural network structure consisting of curved fin angle, Reynolds number and heater plate temperature parameters is preferred. Heat transfer is estimated by feedforward backpropagation (FFBP) and multi-layer perceptron (MLP) neural network algorithm using a data set containing 45 empirically obtained values in the forced convection condition. The training network has been calculated through Scaled Conjugate Gradient with ten neurons in the hidden layer. The results obtained from the experimental data assessed in consequence of the comparison with the target value within the ±4% diversion interval for all Nusselt numbers. Optimum mean square error (MSE), average relative deviation (ARD%) and correlation coefficient ( R 2 ) is obtained as 1.4x10 -2 , 0.1,771 and 0.9879 respectively in architecture of average Nusselt number datasets.

Prototype of an experimental stand for investigating heat and moisture transfer phenomena in building materials

The prototype of an experimental stand for studying drying building materials is presented in the paper. The stand is designed to validate a numerical model of heat and moisture transfer in porous media. The experiment was conducted in forced convection conditions. A building material sample was located in a holder so that only one surface (the top one) had contact with flowing air in the duct. The sample exchanged heat and moisture with the flowing air through this surface. Two fans at the end of the duct induced the airflow through the vent. The stand was placed in a climatic chamber, stabilizing the temperature (22.5°C) and relative humidity (50%) during the experiment. A tensometric beam was used to measure the temporal variation of the sample mass. A resistance thermometer (PT100) was used to measure the temperature on the sample surface. Four samples (90×90×30mm each) from cellular concrete were prepared. Each sample was dried before the experiment, immersed in water for 24h, then wrapped in the foil and preconditioned (24h) in a climate chamber to equalize the temperature. The samples were dried on the stand for 48 hours. The procedure was repeated twice. The repeatability of results has been achieved. However, the accuracy of the experimental stand was insufficient. Hence further modifications need to be made were proposed.

Анализ длительно допустимых токов и потерь активной мощности в воздушных линиях электропередачи с учетом климатических факторов

Formulas for calculating continuous admissible currents and active power losses for overhead power transmission lines under conditions of natural and forced convection for insulated and non-insulated conductors are presented. The correct determination of maximum current loads plays an important role in the efficient use of the load capacity of power transmission lines, and the refinement of the calculation of active power losses is the basis for increasing the efficiency of energy transmission. The results obtained can be used in the selection of measures aimed at increasing the capacity of overhead lines and reducing energy losses.

Results of the calculation of the absorber temperature in a flat solar air heater

The article discusses a method for calculating the temperature of the solar air heater absorber under conditions of still air in the collector and under conditions of low forced convection. The problem of determining the temperature of the air heater absorber tst is considered for the climatic conditions of Fergana, with a horizontal collector and when solar radiation passes through double glass. It is assumed that over the considered short period of time, the temperature of the absorber is constant and the heat loss through the insulation of the bottom of the collector is negligible. Solar radiation passes through ordinary transparent colorless glass with a reflectance of 8%. Taking into account the losses of the heat flux of solar radiation when passing through glass 1 and 2 by the reflection coefficients K1 and K2, in the first version the Stefan-Boltzmann equation is solved, as a result of which the temperature value of the absorber surface is obtained. In the second variant, the absorber temperatures were obtained taking into account convection.

Вплив форми гравітаційної теплової труби з різьбовим випарником на її теплопередавальні характеристики

The modern development of electronics is associated with the problem of reducing the mass and size of the equipment while increasing its power. This leads to an increase in the temperature of both individual elements and the device as a whole, which contributes to a decrease in equipment reliability. This makes the development of inexpensive yet efficient cooling systems an urgent task. One of the ways to solve this problem is to use heat pipes of a new simplified design in cooling systems. This article compares the thermal characteristics of a cylindrical copper gravity heat pipe with a threaded evaporator before and after flattening. The working fluid used in the experiment was R141b. Experimental studies were conducted at two tilt angles relative to the horizontal under the conditions of forced convection cooling of the condensation zone. The influence of the change in the shape of the body from cylindrical to flat on the temperature in the heating zone of the heat pipe, thermal resistance, and heat transfer coefficients in the evaporation and condensation zones was shown in the range of heat power from 5 to 70 W. The choice of the heat pipe is justified based on the maximum thermal loads of electronic components and the orientation of the cooling system in space. Changing the shape of heat pipe from cylindrical to flat leads to a decrease in the maximum thermal power transmitted, but a flat heat pipe shape is more technologically feasible for use in cooling systems of electronic equipment.

Исследование процесса испарения водно-этанолового раствора на супергидрофильной капиллярно-пористой поверхности в условиях вынужденной конвекции

In view of the continuous growth of global energy consumption, reduction of non-renewable energy sources, as well as increasing greenhouse gas emissions, it is necessary to introduce pro-mising methods to increase the importance of rational use of energy resources in the industrial, domestic and scientific and technical fields, which will reduce the amount of energy to provide the same level of energy supply of buildings or technological processes in production. This paper considers superhydrophilic capillary-porous surfaces capable of providing heat exchange through phase transition, which is of fundamental importance in evaporative radiators for space applications, heat pipes, as well as in the operation of cooling towers for electricity and heat production in centralized heat supply systems. The study of the heat exchange process with phase transition on the capillary-porous surface was carried out by conducting experiments on evaporation of water-ethanol mixture of different concentration under conditions of forced convection at diffe¬rent orientation of the modified surface in space. The results obtained in this work allow us to judge about the effectiveness of capillary-porous structures in heat exchangers operating on eva¬poration cycles and the Maisotsenko cycle, and are also applicable to further study of heat and mass transfer processes occurring on superhydrophilic capillary-porous surfaces.

Thermoacoustic waves and Piston Effect inside a microchannel with Carbon Dioxide near critical conditions

• Adiabatic thermalization inside microchannels. • Thermoacoustic waves inside microchannels. • Computational Fluid Dynamics study of Piston Effect ( PE ) under terrestrial and Zero-gravity conditions. • Existence and Quantification of PE in forced convection. • PE in near-critical carbon dioxide. • Comparison of acoustic time scales and diffusion time scales. Heat transfer near the critical condition of Carbon Dioxide due to thermo-acoustic waves in a 100-µm high microchannel was numerically studied. The fluid temperature outside the thermal boundary layer was compared between computational fluid dynamics (CFD) models and a pure conduction model. The comparison revealed that the CFD model predicted a temperature increase in the bulk fluid much faster than the time constant required for such increase purely by conduction. It is believed that another heat transfer process, termed the piston effect ( PE ), which is associated with pressure waves in the fluid, was responsible for this increase. Explicit unsteady fluid model indicated that propagation of pressure waves due to the rapid expansion of the boundary layer and the associate change in the fluid density distribution resulted in this temperature rise. It was confirmed that natural convection wasn’t responsible for the temperature increase under quiescent conditions and it was revealed that the PE time scales are 5 to 6400 times faster than diffusion time scales. In addition, it was discovered that the PE is significant for certain forced convection conditions that transfer 5-7% of the total power input to the bulk fluid. A new correlation was proposed to correlate the temperature rise in the bulk fluid in terms of Fourier and Reynolds numbers.

Numerical study on channel-flow convection heat transfer enhancement with piezoelectric fans under various operating conditions

Piezoelectric fans have been widely considered for electronics cooling as a stand-alone cooling device with its advantages over conventional rotary fans, such as low power consumption, low noise level, and small form factor. However, the application of the piezoelectric fans as a supplemental device in an existing forced-convection condition is rare in the literature. In this study, a transient numerical simulation is carried out to investigate the impacts of the piezoelectric fan, operating at various conditions and locations, on the convection heat transfer of a heated surface in an airflow channel. The mean-to-peak displacements and the operating frequencies of the piezoelectric fan are varied from 3.4 to 7.8 mm and from 90.3 to 180.6 Hz, respectively, for case studies. As a result, when the piezoelectric fan is operated at the front-end location with its maximum displacement and frequency, about 70 % of heat transfer improvement is achieved over the channel flow condition without the piezoelectric fan operation. Furthermore, the Q-criterion analysis is used to visualize vortical structures generated by the piezoelectric fan operating at three relative locations from a heated surface. It is found that the vortical structures carry most of the kinetic energy, creating a significant enhancement of the convective heat transfer in the channel.

Mesoscopic-scale study of convection-induced morphological evolution of solid-air dendrites in liquid hydrogen

The trace amount of air infiltration that may occur during the utilization of liquid hydrogen may be confronted with a severe security risk. Considering the effect of convection, a six-fold symmetric solid-air dendrite growth numerical model is established for the coupling of flow field, temperature field and solute field during solidification process. For regular Cartesian grids, a factor is introduced to reduce the anisotropy of grids, and its validity is verified. The reliability of the model is evaluated by comparing the numerical results of the growth of ice-dendrite with the same face-centered cubic structure with the experimental results, and the subsequent numerical results of solid-air dendrites growth rate match well with the results of analytical theory. On this basis, the growth characteristics of solid-air dendrites under diffusion condition and natural convection and forced convection conditions are studied. The results show that the tip of each dendrite arm has a consistent growth rate under diffusion condition, and the dendrites present a six-fold symmetric morphology. The dendrite growth is promoted in the upstream and inhibited in the downstream under both natural convection and forced convection conditions, and the morphology of the reshaped dendrites lose symmetry. Forced convection promotes upstream more than it inhibits downstream. The mechanism of the two conditions affecting the growth of solid-air dendrites and the differences between them are explained in detail.

Numerical Simulation of Transport Phenomena in Directional Solidification Castings with Changeable Cross-Section and Solidification Interface Control

The roles of traveling magnetic fields (TMFs) within the transport phenomena during the directional solidification of nickel-based superalloys were simulated. The evolution of thermal field, flow field and solid-liquid interface morphology during the solidification process under both natural and forced convection conditions were also simulated and compared. The strength of TMFs window that suppresses the flow of the interfacial front in the melt was quantified. The association between flow velocity at the interface front and defect formation was discussed.

Vortex shedding controlled combustion of the wake flame of an n-heptane wetted porous sphere

Aiming at the combustion characteristics of the microscopic flying fuel drops in fuel sprays in engines, an n-heptane wetted porous sphere (d0 = 6 mm) was employed to simulate the combustion of the fuel drops according to the flow similarity indicated by the Reynolds number (Re). The hot airflow provided by a wind tunnel with an inner diameter of 40 mm varied in flow velocity from U = 0.6 to 7.1 m/s and from temperature T = 423 to 723 K, with Re ∈ [0, 800]. Experimental results show that flow velocity plays a dominant role in determining the morphologies and fluctuation frequencies of the wake flame. The flame length and suspension height have a trade-off relationship and approach a linear relationship with the increase in air temperature. With increasing Re, the yellow flame gradually vanishes, and the blue flame ratio in the total flame increases following a hyperbolic tangent function. In addition, the length of the flame outline increases linearly with Re. The fluctuations in the flames under both natural and forced convective conditions and various air temperatures are proved to be determined by the vortex shedding behind the sphere, which helps one understand the underlying physics of spray combustion better.

Examination of Twisted Elliptical Tube Heat Enhancement Within the Mixed Convective Zone

Previously, the investigation of the heat transfer performance of twisted elliptical tubes has been restricted to the effects of forced convective conditions without considering the impact of buoyancy conditions. Therefore, this work investigates heat transfer enhancement of twisted elliptical tubes of different pitch lengths at different mixed convective regimes. The model continuity, momentum, and energy equations were solved using the finite volume technique and validated using both analytical and experimental results. Under the variation of Gr and Re, the results show that twisted elliptical tubes are better than smooth elliptical tubes for heat transfer but with a corresponding high drag coefficient. The heat enhancement of 48% was obtained at a twisted pitch length of 200 mm and Re of 2000 as compared to the plain tube. In addition, on the effect of twist pitch length, the results show that the more twisted a tube is the more its Nu and F. This indicates that induced secondary flow by the twist has a high impact on the Nu and F. The results show that heat enhancement of the elliptical tube depends on P, Gr, and Re. Similarly, there is a fluid-thermal-structure interaction interplay in the hydrothermal behavior of elliptical tubes.

The electrochemical behaviour of suspended Prussian Blue nanoparticles in forced convection conditions

• Electroreduction of Prussian Blue nanoparticles at RDE is described by Levich equation. • Minor fraction of suspended Prussian Blue nanoparticle is reduced at RDE. • Electrochemical detection of suspended Prussian Blue nanoparticles in flow. The electrochemical studies of suspended nanoparticles became an important method of their characterisation. In reported research, Prussian Blue nanoparticles served as a model redox active nanoobjects for studies in forced convection conditions. Recently, (M. Holdynski et al. Electrochem. Commmun. 86 (2018) 130–134), we have demonstrated that injection of these nanoparticles suspension into electrochemical cell results in a sharp decrease or increase of the current at glassy carbon rotating disc electrode set at potential corresponding to their electroreduction or electrooxidation respectively. Here, on the example of the electroreduction of Prussian Blue nanoparticles, we show that the shape of potential vs time dependence is independent of the electrode rotation rate. Analysis of the current–time dependences based on the Levich equation indicates that only an average 0.12 % of Fe 3+ ions present at the surface of the given nanoparticle undergoes electroreduction in the elementary reaction step. Injection of Prussian Blue nanoparticles into flowing aqueous electrolyte produces a sharp increase of the current recorded at glassy carbon electrode of the electrochemical wall-jet detector, followed by its gradual decrease. Such behaviour is generally observed for redox-active molecules. The ratio of the peak currents corresponding to electrooxidation and electroreduction of Prussian Blue nanoparticles colliding with the electrode differs from the expected from the stoichiometry of reactions.

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.

Influence of oblique angle variations on the thermo-hydraulic characteristics of the oblique fin heat sink with Al2O3 water nanofluid

The flow mixing through secondary flow is critical for improving the heat transfer of microchannel heat sinks. This study investigates oblique fin heat sinks (OFHSs) under forced convection conditions with Al2O3-water nanofluid under the Re 100–500. The study is referenced with a straight channel heat sink for benchmarking purposes. Five oblique angle configurations (15°, 25°, 35°, 45°, and 55°) were investigated at the constant fin pitch and constant width ratio (primary width-to-secondary width ratio) of 2:1. The RNG k-ε model has been employed with enhanced thermofluidic effect and wall treatment. The findings revealed that secondary flow generation leads the thermal boundary layer to redevelop at each fin, resulting in continuous fluid flow development. Pressure drop increment has been noticed with oblique angles 15° to 25°, and the variation is almost insignificant among the 35°, 45°, and 55° OFHS. The secondary flow rates were found to have a profound effect in all the cases. The 25° degree shows a higher average convection coefficient among all the OFHSs. Also, the convection coefficient varied in the following order for both water and nanofluid: 15° < 25° > 35° > 45° > 55°. The 25° OFHS enables a more uniform water/nanofluids temperature to build up in the streamwise direction than the 35°, 45°, and 55° OFHS. The thermofluidic analysis suggests that the proposed designs have superior heat convection performance than their flow resistance penalty. Therefore, the 25° OFHS offers a promising option for further parametric research in the future.

Experimental Investigations on Thermal-Hydraulics of Supercritical Carbon Dioxide Under Natural Circulation Vertical Flows

Abstract Advanced energy systems propose use of supercritical fluids (SCFs) as working fluids owing to various advantages associated with SCF. Supercritical water (SCW), supercritical carbon-dioxide (SC–CO2), and supercritical helium (SC–He) find proposed applications in advanced nuclear reactor designs too. Heat transfer behavior of SCFs is still not understood completely. Though there exist many heat transfer correlations and criterion in the literature, they miss having a general consensus among them. In spite of having many reported studies in literature for SCFs under forced convection conditions, studies related to SCF under natural circulation (NC) mode are very limited. In the present study, extensive experiments have been performed with SC–CO2 under NC conditions in tube geometry. The experimental data generated for vertical heated test section at various pressures between 8.0 and 13.0 MPa have been utilized to bring out salient findings related to fluid flow and heat transfer. The data have also been utilized to make comparison with generalized NC flow correlation available in the literature. Besides, few popular heat transfer correlations have been compared with experimentally obtained heat transfer coefficients.

Machine learning approach to predict the heat transfer coefficients pertaining to a radiant cooling system coupled with mixed and forced convection

Mixed convection phenomenon over radiant cooled surfaces with displacement ventilation in living environments is becoming a popular issue due to the airborne viruses and energy economy. Artificial neural networks are one of the machine learning methods that are widely evaluated as an engineering tool. In the current study, heat transfer coefficients for a radiant wall cooling system coupled with mixed and forced convection have been predicted by a machine learning approach. This approach should be noted as a first experimental investigation couple with an artificial neural network analysis in the open sources in which mixed convection systems in real sized living environments is examined. Experimentally obtained heat transfer coefficients have been used in the development of the feed forward back propagation multi-layer perceptron network structure. So as to analyze the impact of the input factors on the prediction performance, two neural network structures with dissimilar input parameters such as various temperatures, velocities, and heat transfer rates have been developed. By means of feed forward back propagation multi-layer perceptron neural network algorithms, convection, radiation, and total heat transfer coefficients have been predicted using the experimentally acquired dataset including 35 data points belonging to the mixed and forced convection conditions. Training, validation, and test data groups include 70%, 15%, and 15% of the dataset, in turn. Training algorithm has been computed via Levenberg-Marquardt one with 10 neurons in the hidden layer. The findings obtained from the computational solution have been evaluated as a result of the contrast with the target data with in the ±5% deviation band for all heat transfer coefficients. The performance factors have been computed and the estimation precision of the numerical models has been thoroughly examined.

Controlling parameters and regimes for preferential vaporization of jet fuel droplet with liquid transport and convection

Petroleum-based fuels and some biofuels contain hundreds of compounds with a wide range of volatilities. Vaporization of such fuels usually favors more volatile species throughout the process. Such preferential vaporization has prevailed in practical applications, such as direct-injection engine and gas turbine. To numerically study preferential vaporization of a liquid complex fuel, a 24-component jet fuel (RP-3) surrogate was used in the present study. The “film theory” and “effective thermal conductivity/effective diffusivity” (ETC/ED) for droplet vaporization considering forced convection were employed to study the preferential vaporization of jet fuel droplets. To quantitatively assess the preferential vaporization, degree of preferential droplet vaporization was quantified using the reciprocal of gasification of Peclet number. Three environmental factors of preferential droplet vaporization were investigated in this work to find the controlling factors of droplet preferential vaporization: ambient temperature (573–1073 K), ambient pressure (1–20 bar) and forced convection (0–6 m/s). The results show that the preferential droplet vaporization is determined by the combined action of droplet vaporization rate and liquid transport. Specifically, under different environmental factors, the controlling parameters for preferential vaporization are different. Under different ambient temperatures, the preferential vaporization is highly affected by the droplet vaporization rate. Under different ambient pressures, the preferential vaporization is determined by the liquid transport through the liquid Lewis number. Under different forced convection conditions, both vaporization rate and liquid transport are enhanced by forced convection, whose effects on droplet preferential vaporization are in competition. In such cases, two distinct regimes separated by a critical liquid Reynolds number are recognized, each of which exhibits independent controlling scheme for droplet preferential vaporization.

Efficiency of Bimetallic Ribbed Tube Bundles in Forced Convection Conditions

Efficiency of Bimetallic Ribbed Tube Bundles in Forced Convection Conditions

Використання регулярного теплового режиму для прогнозування інтенсивності теплообміну у відходах птахівництва

The system “environment (water in an annular volume) — a thin cylindrical metal wall — the investigated liquid medium” is being studied. Heat exchange during heating and cooling of water-diluted chicken litter under conditions of forced convection is studied. Chicken droppings represent a multiphase medium, as it consists of solid particles based on a liquid medium. The research is carried out on an experimental stand developed at the Department of Thermal Power Engineering of the National Technical University of Ukraine. Experiments are conducted under conditions of forced convection. Average temperatures are defined as arithmetic averages, based on the results of temperature measurement by thermocouples on vertical thermoprobes at a certain point in time. Processing is carried out by the method of stationary heat exchange, examining the entire period of time. After conducting an analysis of the distribution of excess temperature over time Lnϑ = f(τ) during the study of heat exchange of chicken litter under the conditions of its cooling and heating, a linear rate of cooling/heating m = const was determined, which is characteristic of a regular thermal regime during cooling/heating of a solid body (system of solid bodies) and maintaining the stability of the ambient temperature. It was experimentally established that there is a regular thermal regime in the experimental system: the rate of cooling/heating is constant, m = const; the heat transfer coefficient during the regular thermal regime is practically constant. The coefficient of heat transfer from the inner wall of the cylinder to the studied environment was determined by the method of regular thermal regime and the calculation-experimental method. After analyzing and comparing the heat transfer coefficients calculated using the calculation-experimental method and found by the method of regular thermal regime, a discrepancy of 20 % was established. It was established that, the method of a regular thermal regime can be used to determine the intensity of heat exchange in multiphase media.