Total Heat Transfer Coefficient Research Articles

Experimental investigation of mixed and forced convection generated by displacement ventilation with radiant wall heating

There is a rising necessity concerning low energy consumption and thermal comfort, thus, new systems combining displacement ventilation and radiant heating are becoming common as a research topic. The current paper covers free, mixed, and forced convection in an experimental chamber consisting of a radiant wall heating system and a baseboard level diffuser. Air temperature in the test room is adjusted using an air blower via a novel designed baseboard level slot diffuser along with a hydronic wall heating circuit. Simultaneously; an upward direction air jet is blown through the heated or the opposite wall under two scenarios. The ranges of air inlet velocity and temperature are selected to be between 0.25 and 12.5 m/s and 14–26 °C, correspondingly. The heat transfer characteristics pertaining to the heated wall are studied with respect to the influences of wall and air temperatures, air inlet velocities, and the location of the diffuser. To derive convective heat transfer coefficient correlations valid for mixed and forced types for a radiant heated wall, acquired data have been processed. The convective, radiative, and total heat transfer coefficient intervals of 3–14.1, 5.2–5.5, and 9.3–21.7 W/m2K have been obtained for the radiant heated wall coupled with airflows, correspondingly.

Effect of cooling water flow on heat transfer performance of horizontal tube spray falling film evaporator in ORC system

Organic Rankine cycle (ORC) uses low-temperature heat source to realize thermoelectric conversion, which is an important way to realize the effective recovery and utilization of waste heat. Based on the horizontal tube spray falling film evaporator, an orc power generation system with R245fa as the circulating working medium is built to study the influence of the change of cooling water flow on the heat transfer performance of the evaporator. The results show that under the condition of constant heat source temperature, the evaporation pressure and condensation pressure first decrease and then increase with the increase of cooling water flow, while the total heat transfer coefficient outside the evaporator tube first increases and then decreases. When the cooling water flow is greater than 3100 L/h, it has little effect on the heat exchange system of the evaporator, and it is more appropriate to keep the cooling water flow at about 3100 L/h. By studying the influence of cooling water flow on the heat transfer performance of evaporator, an important experimental basis is provided for the design and optimization of ORC cooling system.

An experimental investigation on the radiant cooled wall’s heat transfer characteristics in a fully conditioned real-sized living environment

Radiant wall applications have been favored compared to floor and ceiling ones owing to their advantages. In the current work, the heat transfer characteristics of a radiant cooled wall, which are heat fluxes, and heat transfer coefficients, have been experimentally investigated by altering supplied water temperatures and using proper reference temperatures in a fully conditioned real-sized test chamber where the obstacles such as unrealistic room sizes and insufficient air conditioning processes have been eliminated. Detailed methodology for the determination of reference temperature, used thermocouples’ locations, and heat gains were revealed. As a result, investigated radiant cooled wall’s average total, radiative, and convective heat transfer coefficients were determined as 7.83, 5.14, and 2.57 W/m2K, in turn. 67 % of the total heat transfer rate in the mean was found to belong to radiation. Furthermore, novel empirical correlations on the total, radiative, and convective heat transfer coefficients were derived with the mean squared errors of 99 %, 98 %, and 99 %, respectively. There is no experimental investigation on radiant wall cooling excluding authors’ previous publications. Finally, a detailed measured and calculated dataset, obtained from the third investigation on the radiant cooling wall system in open sources, was presented for the sake of other researchers.

Analysis of the effects of forced convective heat transfer to reduce the efficiency of heaters of gas pressure reducing stations

This study investigates the thermal modeling of heaters at the city gate stations (CGS) in Iran with a focus on efficient heat transfer and fuel consumption. Gas heaters are basic equipment with applications in various industries, especially CGS sites. Research on gas heaters and their optimal and efficient performance has been a challenging subject in the engineering world. This study aimed to change the heaters' thermal functionality from free to forced convection and examine the effect of such alteration on the heaters' efficiency while reducing their energy consumption. To make forced convection in the heater, a pump is required to be installed on it. This pump causes fluid circulation inside the heater and by creating movement in the fluid, it increases the heat transfer coefficient and changes the heat transfer mechanism of the system from free convection to forced convection. Using the first law of thermodynamics, the net energy these heaters emit into the environment consists of multiple components. These include the energy expended as follows: in the chimney, to warm up the gas, and the part dissipated away from the heater's body. The heat transfer coefficient is determined after calculating the energy loss for each of the components. Changes in the required energy for the total heat transfer coefficient for both free and forced convection may be analyzed. The results of this research indicate that forced convection heat transfer causes more heat loss from the heater's body than that transferred to the gas stream in the heater coils. This energy transfer ultimately reduces the heater's efficiency concerning the net generated heat. Thus, a reduction in the efficiency occurs and the initial efficiency (32%) of the free convection declines to a lower coefficient value for the forced convection.

Experimental examination of heat transfer coefficients in hydronic radiant wall cooling systems

Radiant heated and cooled wall systems may offer benefits like building retrofitability, architectural flexibility, a comfortable thermal atmosphere, and compatibility with alternative energy sources. In certain situations, radiant wall systems are preferable to floor and ceiling systems due to their benefits. In this study, a set of tests were performed in order to assess the heat transfer features of the hydronic radiant wall cooling system. A climatic test chamber was constructed for this objective, and convection, radiation, and total heat transfer coefficients (HTCs) were evaluated and analyzed. For the purpose of calculating the HTCs, experiments were conducted at various supplied water temperatures. The HTCs were then estimated using the corresponding characteristic temperature differences. The total, radiation, and convection HTCs for the cooled radiant wall were found to have estimated mean values of 8.25, 5.52, and 2.41 W m−2.K−1, respectively. Radiation contributes to around 70% of the overall heat transfer on average. Additionally, two different radiant panel arrangements were tested to examine how the thermal conductive layer influenced the thermal performance of the cooled radiant panels. The cooling capacity of the panel with an aluminum conductive layer is about 21% more than that of the traditional panel.

Condensation Heat Transfer Correlation for Micro/Nanostructure Properties of Surfaces.

Condensation, which can be observed in nature as a phase change heat transfer phenomenon, is a critical phenomenon in industrial fields such as power generation, water desalination, and environmental control. Many existing studies have applied surfaces with different wettability by controlling the surface topology to enhance condensation heat transfer. However, the industrial applicability is close to zero due to the limited size and shape of surfaces and low supersaturation conditions. Here, we regulate the surface topology of large-area copper tubes, which are representative industrial metals. We fabricated four copper tubes with different surface structures. We analyzed the condensation phenomenon of the modified tube under specific supersaturation conditions by measuring the overall heat transfer coefficient. We analyzed the condensation phenomenon by measuring the condensation heat transfer coefficient. We have recognized that there is a difference between the maximum droplet radius and the droplet detaching frequency depending on the size and shape of the structure. We measured the contact angle and contact angle hysteresis to accurately analyze the droplet behavior on each surface. As a result, we show that there is a correlation between contact angle hysteresis (CAH) and the total heat transfer coefficient, indicating heat transfer performance. These findings can be applied when evaluating surfaces with excellent condensation heat transfer performance for use in real industrial environments, which can dramatically reduce time and cost.

Theoretical and numerical analysis of conjugate heat transfer for supercritical CO2 flowing in printed circuit heat exchanger

The printed circuit heat exchanger (PCHE) is often utilized for the supercritical CO2 (SCO2) Brayton cycle. This study proposed a assessed method with the dual effects of temperature and pressure to investigate the heat transfer performance (HTP) for PCHE derived from theoretical analysis and numerical validation. The influence of various operating conditions on total and hot-side heat transfer coefficient (HTC) was analyzed. The results showed that the PCHE may achieve better HTP when the critical index was near 1. The assessed way had been verified by utilizing the existing experimental data. The total and hot-side HTC for SCO2-Water PCHE were higher than that of SCO2-SCO2 PCHE, which may increase as cold-side Reynolds number increases. The effect of entropy generation rate for heat transfer on the irreversibility was larger than that of entropy generation rate for viscosity. This study may offer theoretical guidance for optimization of heat exchangers.

Design and optimization of a solid oxide fuel cell-inverted gas turbine integrated system with zero carbon emission for distributed cogeneration

Solid oxide fuel cell-gas turbine (SOFC-GT) hybrid system is an efficient and potential method for distributed energy supply. The system layout, component operating parameters, system-level heat integration, component selection and design, etc., determine the system performance. To further improve the performance and achieve zero CO2 emissions, a ambient-pressure solid oxide fuel cell-inverted gas turbine (SOFC-IGT) hybrid system with CO2 capture using oxy-fuel combustor was proposed. With the aid of partial-load component model and a multi-objective optimization method established, the heat exchanger network and heat exchanger physical structure were designed and optimized. The results show that, pursuing a higher gross electrical efficiency leads to a decline in heat output. The electrical efficiency can be improved by increasing the external reforming ratio while ensuring self-sufficiency in heat. The heat integration is critical to enhance system performance, and optimizing heat exchanger network can make full use of the heat of stack exhaust and reduce the demand for external cold utility. The heat exchanger structural design has a trade-off between the total heat-transfer coefficient and the pressure drop. The net electrical efficiency of the three special solutions can reach 53.9 %, 65.7 %, and 65.8 %, respectively. The design of the maximum heat point (MHP) scheme can be considered when pursuing lower cost, and the design of the maximum power point (MPP) scheme can be employed for higher performance.

The Effect of Microencapsulated PCM Slurry Coolant on the Efficiency of a Shell and Tube Heat Exchanger

This paper describes the results of experimental studies on heat transfer in a shell and tube heat exchanger during the phase changes of the HFE 7000 refrigerant. The studies were performed using a mixture of water and a microencapsulated phase change material as a coolant. HFE 7000 refrigerant condenses on the external surface of the copper tube, while a mixture of water and phase change materials flows through the channels as coolant. Currently, there is a lack of research describing cooling using phase change materials in heat exchangers. There are a number of publications describing the heat exchange in heat exchangers during phase changes under air or water cooling. Therefore, the research hypothesis was adopted that the use of mixed water and microencapsulated material as a heat transfer fluid would increase the heat capacity and contribute to the enhancement of the heat exchange in the heat exchanger. This will enable an increase in the total heat transfer coefficient and the heat efficiency of the exchanger. Experimental studies describe the process of heat transfer intensification in the above conditions by using the phase transformation of the cooling medium melting. The test results were compared with the results of an experiment in which pure water was used as the reference liquid. The research was carried out in a wide range of refrigerant and coolant parameters: ṁr = 0.0014–0.0015 kg·s−1, ṁc = 0.014–0.016 kg·s−1, refrigerant saturation temperature Ts = 55–60 °C, coolant temperature at the inlet Tcin = 20–32 °C, and heat flux density q = 7000–7450 W·m−1. The obtained results confirmed the research hypothesis. There was an average of a 13% increase in the coolant heat transfer coefficient, and the peak increase in αc was over 24%. The average value of the heat transfer coefficient k increased by 5%, and the highest increases in the value of k were noted at Tin = 27 °C and amounted to 9% in relation to the reference liquid.

RESEARCH OF THE THERMAL REGIME OF THE BELT CONVEYOR MOTOR-DRUM

The paper presents a method for calculating the maximum power of the conveyor drive under the condition of the maximum heating temperature of the elements. To study the thermal regime of the drive, the drive was selected according to the motor-drum scheme. The peculiarity of this drive is the lack of fan and air cooling, which can lead to overheating. The main components that generate heat energy are an induction motor with a short-circuited rotor and a reducer. The space of the motor-drum is filled with working fluid, which performs the functions of lubrication and cooling of the system components. Analysis of the design of the motor-drum drive allowed to establish the quantitative characteristics of the sources of thermal radiation of the individual components of the drive, taking into account the peculiarities of the interaction of the components. Quantitative characteristics of the sources of thermal radiation of the individual components of the drive allowed to establish the mutual influences of different parts of the object of study on its temperature state. The heat balance equation is applied under the condition of uniform distribution of the heat field on the surface of the drive elements. The equation of thermal balance of the system motor-drum-environment allows to determine the limit values ​​of the drive power at known geometric parameters of the system. Coefficient of convective heat transfer coefficients between the working medium and the drum wall and the drum wall and the environment are used to determine the total heat transfer coefficient of the working medium-drum-environment system. Analysis of the obtained data shows a significant limitation of the required drive power. Limited drive power does not allow this type of drive to be used for long conveyors. Increasing the power of the drive according to this scheme can be provided by forcibly cooling the working fluid or changing the design of the drive by increasing the surface in contact with the working medium.

Analisis Perpindahan Kalor Pada Double Pipe Heat Exchanger Beraliran Lawan Arah Menggunakan Sirip Trapesium Dengan Fluida Cair

<p><em>This study aims to analyze the counter flow performance of a double pipe heat exchanger using an experimental method with the influence of the flow rate of hot and cold substances. The results showed that the counter-flow double-pipe heat exchanger produced a total heat transfer coefficient of 69.229 W/m2</em><em>℃</em><em>. The double pipe heat exchanger produces Reynolds number of 82.17 and Nusselt number of 1.18 in hot fluids, while in cold fluids it produces Reynolds number of 4.528.42 and Nusselt number of 31.52. The double pipe heat exchanger has an effectiveness rate of 116.16%.</em></p>

Evaluate the hydrothermal behavior in the heat exchanger equipped with an innovative turbulator

New copper generators were designed and manufactured to enhance the heat transfer of the double-tube heat exchanger and raise the thermal performance. The copper tube side of the heat exchanger is manufactured with inner and outer diameters of 0.02 m, and 0.022 m, respectively. While Perspex shells manufactured with inner and outer diameters of 0.054 and 0.06 m, respectively, were used. In this study, a cut square turbulator tape was modeled with three different dimensions of square fins, and a hydrothermal examination was performed on them. Three square pieces of different turbulator bars (1 cm, 0.5 cm, 0.25 cm) were selected. The effect of the inclusion of the new turbulator strip on thermal performance, friction factor, and overall heat transfer coefficient was calculated and compared with conventional and other improved heat exchangers. Reducing the width of the turbulator strip fins significantly affected the heat transfer rate in turbulent flow conditions. Compared with the smooth tube, the experimental results showed that the 0.25 cm wide square cut turbulator causes higher Nu number values by 271.7% while for total heat transfer coefficient of 117.6% and 113.7% increase in hot water temperature difference. Efficiency and TPF showed higher values of 2.1 and 2.9 in the case of turbulator with 0.25 cm Square-cut width. The newly designed pumps were compared with other improved heat exchanger works and showed very promising results.

Flexible stretchable electrothermally/photothermally dual-driven heaters from nano-embedded hierarchical CuxS-Coated PET fabrics for all-weather wearable thermal management

The multifunctional photoelectronic devices are recently attracting much more attention due to their potential enlarged applications. The flexible stretchable electrothermally/photothermally dual-driven heaters for all-weather wearable thermal management are presented in this work with nano-embedded hierarchical CuxS-coated PET fabrics. Herein, the hierarchical nano-embedded CuxS film is fabricated via a simple chemical bath method for high electrical conductivity and highly efficient inelastic collision of electro/photo-generated carriers. The hierarchical nano-embedded CuxS morphology produces the low sheet resistance of 1.26 Ω sq-1 and the super low total heat transfer coefficient of 3.256 × 10-5 W/oC·mm2, which lead to the high-efficient electro/photo-dual-driven heating effect in the CuxS@PET fabrics. The saturated temperature on the as-fabricated flexible wearable heaters reaches up to 172 °C. The thermal conversion devices also bear the excellent stability, reproducibility, stretchability, controllability and corrosion-resistant characteristics. Interestingly, their excellent thermal conversion performance could be achieved by freely exchanging the driving power sources, such as electricity-supplying equipment, 635-nm laser, infrared physiotherapy lamp and solar simulator, which provides a necessary precondition for the all-weather applications of flexible wearable heaters. The as-fabricated electro/photo-dual-driven heaters on the CuxS@PET fabrics have the promising applications in wearable electronics, all-weather self-heating facilities, out/in-vivo physiotherapy, and so on.

Investigation on Shell and Tube Heat Exchanger by Using CFD

In this study, a detailed technique is built to examine the effectiveness of a shell and tube heat exchanger (STHE) with 35% baffle cuts (Bc) and variable numbers of baffles. Water was used as the working fluid in CFD simulations of a single pass and single tube heat exchanger. For this simulation investigation, a counterflow approach is used. The commercial CFD software package ANSYS-Fluent was utilized for computational analysis of STHE, and distribution of temperature was obtained due to the usage of copper and aluminum materials. In this research, the shell and tube heat exchanger was studied with the number of baffles of four, and six pieces by varying the mass flow rate value which is 0.5,0.8, and 0.9. From this research, it was found that the highest total heat transfer rate and highest heat transfer coefficient is produced by copper with Six baffles at 0.9 mass flow rate.

An experimental study on the heat transfer characteristics over a radiant cooled wall exposed to mixed and forced convection driven by displacement ventilation

An increasing demand on energy efficiency and indoor air quality exists, therefore displacement ventilation and radiant surface cooling applications are taking more attention as an investigation subject. This article presents an experimental study of mixed convection in a test chamber equipped with both radiant wall cooling and air-conditioning systems. The room is conditioned by a hydronic circuit with embedded pipes in the wall and an air jet supplied by a newly designed baseboard longitudinal slot that is apart from relevant literature. The tested radiant wall temperature is kept around 14 °C. Meantime; a parallel upward directional air jet is applied over the cooled wall or the opposite wall. The air jet velocity and its temperature lie within the intervals of 0.25–10 m/s and 18–28 °C, respectively. The natural, mixed, and forced convection heat transfer characteristics over the wall have been investigated considering the effects of radiant cooling, airflow rates, and ventilation location. The measured data are used for the derivation of mixed and forced average convective heat transfer coefficients correlations along a cooled wall. Consequently, the convection and total heat transfer coefficients over the radiant cooled wall are calculated in the ranges of 2–7.3 W/m2K, and 9.1–13.9 W/m2K, respectively.

Comparison and evaluation of supercritical CO 2 cooling performance in horizontal tubes with variable cross‐section by field synergy theory

The cooling/heating performance improvement of supercritical carbon dioxide (sCO2) in heat exchange tube with changing cross-section is significant and crucial for solar-driven advanced thermal systems' application and development. The study introduced and constructed straight, diverging, and converging horizontal tubes with the changing cross-section. The analyzation and evaluation of the sCO2 cooling performance used the combination of SST k-ω turbulence model-based numerical method and the field synergy theory, including the influence of tube cross-sectional shape, inlet pressure and temperature on the cooling performance. As these simulation results indicate, the converging tube can enhance the flow field's synergy and increase the heat transfer ability by 13.15% under cooling conditions. Nevertheless, the heat transfer ability of the sCO2 decreases in the diverging tube under cooling conditions. Besides, the total heat transfer rate rises and the surface heat transfer coefficient has the opposite trends when the inlet temperature increases. Meanwhile, when inlet pressure varies from 11 to 12 MPa, the total heat exchange and surface convective heat transfer coefficient are the maximum. In conclusion, a converging horizontal tube and a suitable inlet pressure can effectively improve the cooling performance of the sCO2. The converging cross-section design will provide an alternative for its application in advanced thermal systems.

Experimental investigation on direct‐contact condensation of subatmospheric pressure steam in cocurrent flow packed tower

AbstractExhaust steam condensation equipment is an important component in thermal power generation systems. Direct‐contact condensation offers high heat transfer efficiency, small flow resistance, simple structure, and less scaling; therefore, its application to exhaust steam condensation equipment is conducive to reducing equipment investment and operation costs. In this study, the direct‐contact condensation of subatmospheric pressure steam (exhaust steam) is investigated in a cocurrent flow packed tower. The effects of steam temperature, steam flow, cooling water temperature, and cooling water flow on the condensation rate, subcooling, number of liquid‐phase heat transfer units (NTUL), and total volume heat transfer coefficient (KV) are investigated. The results show that the direct‐contact condensation of exhaust steam can yield a high condensation rate, low subcooling, and good stability in the cocurrent flow packed tower. Lower steam temperature, higher steam flow, higher inlet‐water temperature, and lower water flow are conducive to the increase in NTUL. It is discovered that NTUL and KV can be expressed by dimensionless parameters of flow and temperature, and that the empirical correlations of NTUL and KV agree well with experimental data.

Flow boiling heat transfer characteristics of two-phase flow in microchannels

A set of experimental platforms with widths of 0.5, 1.0, 1.5, and 2.0 mm was established to explore the mechanism of flow boiling bubble dynamics in microchannels, focusing on heat transfer characteristics, pressure loss, and two-phase flow pattern identification. Bubble flow, restricted bubble flow, and dry area were observed in all four channels. The appearance of flow pattern was related to flow rate and channel width. Under the condition of the same channel width, the initial heat flux of subcooled boiling gradually increased with increase in flow rate, and this change trend was close to the linear trend. Under the same flow rate, the initial heat flux of subcooled boiling increased with decrease in channel width. This condition was due to the faster flow rate of the working medium in the narrow channel, resulting in decrease of heating time. The increase in bubble generation frequency directly led to the increase in the wall heat transfer coefficient and the decrease in the bubble separation diameter. Mathematical analysis showed that under the condition of small flow, reduction of channel size led to reduction of the total wall heat transfer coefficient. In this condition, reduction of channel size cannot enhance heat transfer. With increasing volume flow rate, the range of hydrodynamic control area increased and the index decreased. When the flow rate was large, the total heat transfer coefficient increased greatly with the decrease in channel size. The theoretical values were in good agreement with the experimental data.

Experimental study and mathematical model development for the effect of water depth on water production of a modified basin solar still

Freshwater is constantly shrinking with the confindness of water resources. Therefore, desalination of seawater is given great attention. Renewable energy sources also become of high importance to reduce carbon emissions. Thus, this study is concerned with the design, fabrication, and testing of a new single-slope double-basin solar still as a renewable energy-driven desalination system. It differs from the conventional solar stills by having two basins. The experiments were conducted to compare the performance of both conventional and modified solar stills. As a design parameter that substantially affects the performance, the still water depth was investigated. A new straightforward, accurate model is developed to predict both systems' performance for the design and optimization within a maximum deviation of ±6.6%. The results indicated that the day's productivity for the new and conventional stills at 2 cm water depth was 2.855 and 1.785 L/m2 per day, respectively, by an increase of 59.9% with a thermal efficiency improvement of 61.3%. Also, a rise in the equivalent water depth de from 2 cm to 3 cm reduces the accumulated productivity by 14.36% and 15.41% for the SSDBSS and SSSS, respectively. Additionally, the daily thermal efficiency of the SSDBSS and SSSS is 25% and 15.5% for water depth de of 2 cm, respectively. The maximum values of the total heat transfer coefficient were also evaluated to be 15.4 and 55 W/m2.°C for the lower and upper basins of the modified system, respectively, and 30.18 W/m2.°C for the conventional system.

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.