Heat Transfer Units Research Articles

Experimental study on flow and heat transfer characteristics of three types of finned tube bundle heat exchangers applied in aero-engines

This study outlines the design and manufacture of three types of finned tube bundle heat exchangers (HX) for advanced aero-engines followed by a series of experimental evaluations on flow and heat transfer characteristics. Considering the compactness and reliability, the finned tube bundle HX is more suitable for aero-engines when the heat transfer unit is small-diameter serpentine tubes with the enhanced heat transfer area provided by fins. In this paper, the Logarithmic Mean Temperature Difference method (LMTD) was utilized for the design of a small-diameter (OD: 3.6 mm with 0.3 mm thickness) finned tube bundle HX. Mass reduction was achieved by perforating the fins and removing the support devices, respectively. The flow and heat transfer characteristics of HXs and the impact of the mass reduction schemes were evaluated based on a series of comparative experiments. Among them, the connectionless scheme can significantly improve the power-to-mass ratio, providing it certain application value despite causing considerable flow resistance. Subsequently, empirical correlations for the flow and heat transfer outside the tubes suitable for small-diameter finned tube bundle HXs were proposed based on the experimental results, with over 95 % of the data falling within a 10 % error margin. These correlations have substantially modified the previous one for large-diameter straight tubes, providing an important reference for the future design of HXs in aero-engine.

Hybrid of non-dominated sorting genetic algorithm II and invasive weed optimization for fan duct surface heat exchanger configuration optimization design

In this research, configuration optimization was conducted for FDSHX (fan duct surface heat exchanger) that is a new type of heat exchanger adopted in aero-engine heat management in recent years. Firstly, a method of Taguchi-ANFIS (Taguchi-Adaptive Neuro-Fuzzy Inference System), which can reduce training data volume utilizing orthogonal experimental matrix, was proposed to construct a rapid response mathematical model between three configuration parameters, including fin pitch, fin thickness, and oil passages number, and two design indicators, including heat transfer capacity and operating weight, based on the data prepared by an experimental validated FDSHX heat transfer capacity calculation method using heat transfer unit simulation. Secondly, a hybrid method of NSGA II-IWO (Non-dominated Sorting Genetic Algorithm II-Invasive Weed Optimization), which can simultaneously obtain the strong abilities of exploration and escaping from trap into local optima, was proposed to drive the constructed response mathematical model to enhance heat transfer through adjusting the three configuration parameters. Optimization comparison was performed between NSGA II-IWO and four classic optimization algorithms including GA (Genetic Algorithm), IWO (Invasive Weed Optimization), CA (Cultural Algorithm), and HS (Harmony Search). This research provides an integrated solution to help mechanical engineers improve the applicability and reasonableness of FDSHX design.

Enhanced heat transfer in sandwich heat transfer unit with metal foam: A numerical investigation

In this study, a novel configuration of a sandwich heat transfer unit (SHTU) is proposed, characterized by the partial filling of metal foam on both sides of the fin. The thermal–hydraulic characteristics of the SHTU are investigated numerically using the local non-equilibrium method and Forchheimer-Brinkman extended Darcy model, and compared with those of traditional flat plate-fin heat transfer unit (PHTU). The effect of geometrical parameters, mass distribution of fin and metal foam, and morphological parameters of metal foam on the characteristic of SHTU was explored. Compared to PHTU the Nusselt number of SHTU increases by 13.1% and 27.6%, while the friction factor increases by 6.2% and 33.1% at Reynolds numbers of 100 and 900, respectively. At a Reynolds number of 500, as the filling ratio increases from 0 to 0.95, the Nusselt number, friction factor, and comprehensive thermal performance enhancement increase by factors of 10, 20, and 3, respectively. When the foam fraction increases from 0.30 to 0.90, the thermal performance enhancement improves by 4–5 times within the investigated range. The impact of porosity (ε) and pore density (ω) on the performance of SHTU is not monotonic. The optimal ω was observed to be in the range of 15–20 PPI, while the optimal ε was found between 0.90–0.92.

Experimental study on the influence of the water spray cooling on air-cooled condenser of the split-type air conditioner

The global installed capacity of air conditioners is increasing causing issues over energy consumption. Due to the significant impact of heat transfer at the condenser on the operation of the air conditioning system, there has been a growing interest in improving this process through evaporative cooling. This study experimentally investigates the impact of utilizing water spray on the condenser to enhance the performance of the 12,000 BTU commercial split-type air conditioner. The findings suggest that, in most cases, the spray system reduces the required cooling time to cool the space from ambient temperature to 25 ° C. The performance is influenced by both the nozzle size and the spraying configuration. This was determined by testing three different nozzle sizes and three different layouts. Comparatively, the diagonal design exhibits superior performance to other designs featuring an equivalent nozzle dimension. Moreover, the condensing unit's heat transfer rate is also examined. The coefficient of performance (COP) is also evaluated. The results indicate that the water spray cooling system leads to a significant COP enhancement up to of 6.16 %. Moreover, the implementation of the water spray cooling technology at the condensing unit can lead to a 40 % reduction in energy consumption.

3D-printed triply periodic minimal surface (TPMS) structures as catalyst carriers

The flow and forced convection heat transfer properties of triply periodic minimal surface (TPMS) lattices were investigated both experimentally and numerically. The TPMS structures consist of periodically arranged Gyroid (G) unit cells of 81 % porosity and different unit cell sizes (2–4 mm). This is the first study to evaluate the effect of cell diameter on pressure drop and heat transfer characteristics of solid-gyroid structures prepared by selective laser melting (SLM) technique. Moreover, the range of pore-based Reynolds number covered the laminar and turbulent regimes (Rep=11–1700). The results shown that larger cell diameter provides higher heat transfer coefficients and unit pressure drop. An increase of cell diameter from 2 mm to 4 mm can enhance heat transfer coefficient and unit pressure drop of 39 % and 85 %, respectively. New correlations for flow resistance and heat transfer of solid-gyroid structures were developed. It was confirmed that TPMS lattices can be an alternative to conventional catalytic supports, because they provide comparable thermal efficiency index as foam and packed bed.

Performance prediction and parametric study for annular radiator based on heat transfer unit efficiency and ELM-Sobol’ method

Performance prediction and parametric study for annular radiator based on heat transfer unit efficiency and ELM-Sobol’ method

Study on the flow and heat transfer characteristics inside high-porosity open-cell copper foams: Experimental and numerical explorations

A metal foam with an open-cell structure is a type of material with low flow resistance, high specific surface area, and strong fluid mixing ability. Open-cell metal foams have broad application prospects in electronic component cooling, multiphase heat exchangers, and compact heat exchangers for aerospace applications. This paper presented experimental and numerical analyses of the flow and heat transfer characteristics of five different copper foams under forced air convection. The pores per inch (PPI) of selected foams were 10, 20, 30, 40, and 60, with porosities ranging from 0.968 to 0.973. Analysis of the collected heat transfer and pressure drop data yielded the overall heat transfer coefficient, unit pressure drop, normalized average wall temperature, inertia coefficient, and resistance coefficient. The influence mechanisms of the porosity and flow velocity on heat transfer were analyzed and discussed. The numerical simulation and experiment fitted well. The results showed that increasing porosity led to a significant increase in heat transfer coefficient and unit pressure drop. The 60 PPI foam brought the maximum pressure drop while achieved the minimum thermal resistance.

Reducing Energy Consumption in the Synthesis of Dimethyl Ether (DME) from Methanol Dehydration by Modifying Heat Transfer Unit Using Aspen HYSYS

In the pursuit of a transport sector free from fossil fuels, the incorporation of Dimethyl ether (DME) emerges as a commendable ecological substitute. The DME is a synthetically produced serves as a viable alternative to conventional fuels such as diesel and liquified petroleum gas (LPG). The Dimethyl Ether (DME) production is carried out by catalytic dehydration of methanol over an acidic zeolite-based catalyst. The technological process for the DME synthesis was simulated using Aspen HYSYS based on the combined operating parameters of the reaction dynamic model for the methanol dehydration reaction. This paper attempts to evaluate a modification in dehydration of methanol process to reducing energy consumption by modifying heat transfer unit. The heater and cooler heat transfer units were converted into heat exchangers (HE) by utilizing the output of the process that can be used for other processes so that energy consumption is reduced. The temperature of reaction and the heat transfer unit are modified to reduce energy consumption from 11.850 MMBtu/h to 7.6291 MMBtu/h by changing one heater and one cooler with two heat exchangers. Copyright © 2024 by Authors, Published by Universitas Diponegoro and BCREC Publishing Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Entropy transfer efficiency-effectiveness method for heat exchangers, part 1: Local entropy generation number and operation performance limits

Here, to resolve the open problem of the second-law efficiency evaluation for heat exchangers, an efficiency-effectiveness method is developed from pure convection theory, where the entropy transfer efficiency defined as the outlet temperature ratio of cold and hot fluids exchanging entropy and local entropy generation number can measure the global and local irreversibilities of heat transfer. The efficiency and effectiveness are related by the inlet temperature ratio τ and capacity rate ratio φ, and their maxima subject to the local entropy generation number are determined for different flow arrangements. When τ = φ2, the effectiveness maxima are 0.5/(1 + φ) and 1/(1 + φ) while the highest efficiencies are φ and unity for parallelflow and counterflow arrangements, respectively. The optimum operating points of counterflow exchangers, at which the equal outlet temperature τT1∞ (T1∞ represents the hot inlet temperature) of two fluids with a critical capacity rate ratio τ induces uniform irreversibility, the effectiveness limit of 1/(1+τ), and the efficiency limit of unity, are obtained. A linear temperature difference method is also developed based on the redefined convective and overall heat transfer coefficients, invariably yielding an effectiveness equal to the number of heat transfer units. This method has potentials to be extended to wet heat exchangers with phase-change flows.

Design and optimization of staggered fin structure of heat exchanger based on Machine learning

A multi-scale coupling method based on the unit heat transfer model is proposed to simulate the flow heat transfer performance of a heat exchanger, in view of the complex structure of the heat transfer unit model inside the staggered tooth type heat exchanger and the characteristics of coolant flow. The numerical simulation results are compared with experimental results, and the errors of the simulation results under different working conditions are small, indicating that the modelling method is reliable. On this basis, the heat transfer unit model parameters of staggered fins were established by three-dimensional numerical simulations. The heat transfer performance of the heat exchanger under different structures was compared using a comprehensive evaluation factor, i.e., j/f. The results and experimental data show that the best flow heat transfer performance is achieved with a liquid-side fin length of 7.2 mm, an oil-side fin length of 4.8 mm, a liquid-side fin height of 3.1 mm and an oil-side fin height of 3.1 mm, an inlet flow velocity of 0.2 m/s and a flow angle of 90°. Later, an agent model was established using backward propagation (BP) artificial neural network (ANN) to predict the flow heat transfer performance of the heat transfer unit model, and it was concluded that the f-factor, j-factor and j/f predicted by the ANN model matched well with the simulated values, and the predicted trends of different indicators were the same as the real law, which proved that the error between the predicted and actual values was small, and the above structural parameters were the best architectural parameters, which eventually improved the heat transfer performance by 17 %.

Analysis of operation of heat exchangers of hot water supply system in the scheme of reliable and safe heat supply of residential buildings

The scheme of internal heat supply of residential buildings with independent connection of the heating system to the external heating networks and connection of the heater of the DHW system in a parallel single-stage arrangement, providing reliability of heat supply and the necessary comfort in the premises due to the unrelated regulation of heat supply for heating, ventilation and DHW even in case of fluctuations in water intake, is considered. A mathematical model describing the non-stationary modes of operation of DHW system heaters under the conditions of implementation of this scheme is constructed and investigated. Calculations were carried out using this model, including numerical simulation on a computer using the Monte Carlo method. It is established that in this case, a decrease in the average heat transfer coefficient of the heater of the DHW system due to fluctuations in the flow of mains water following the daily change in water consumption at the DHW can be neglected, since this decrease lies within the usual error of engineering calculation, despite the fact that this coefficient depends on the flow in a nonlinear way, and its growth during the period of increased water intake for DHW does not fully compensate for the decrease during the reduction of water consumption. At the same time, it is proved that with a characteristic number of heat transfer units in a DHW system heater referred to the heated stream, the total amount of heat transferred can decrease quite noticeably, by an amount of 10 to 12 percent; however, with a rational choice of the heater size, such a decrease is within the limits allowed by the applied factors of margin when setting determination of the heat exchange surface.

Dynamic characteristics of refrigerant evaporation temperature in air-water heat source heat pump

In order to improve the problem of reduced heat capacity and unstable heating ability of air source heat pump (ASHP) in the heating mode during the cold season, a heat pump evaporator using air and surface water as a heat source is designed in this paper. A heat transfer unit's calculation model of an air–water heat source evaporator is established. Experimental tests and simulation studies are conducted on the refrigerant evaporation temperature and heat pump heating performance. On this basis, a refrigerant evaporation temperature prediction model is constructed using a multiple linear regression (MLR) model. The trend of refrigerant evaporation temperature under air–water heat source is analyzed by taking the winter meteorological parameters in Xiangtan City as an example. The results show that the air–water heat source heat pump (A-WSHP) has a higher refrigerant evaporation temperature and a smaller temperature fluctuation range than the ASHP. The average values of refrigerant evaporation temperatures under the experimental conditions were 5.3 °C and 4.6 °C, respectively. The temperature fluctuation ranges were 3.8 °C-6.7 °C and 2.8 °C-7.0 °C, respectively. The A-WSHP has higher heating capacity and heating stability. Air temperature, air velocity, water spray temperature and water spray flow rate are all important parameters that affect the refrigerant evaporation temperature of a heat pump. Evaporation temperature changes are positively correlated with these factors. In the refrigerant evaporation temperature prediction model, the air temperature change has the most significant effect on the evaporation temperature, followed by spray water temperature, air velocity, and spray flow rate, whose influence weights are 82.5 %, 38.8 %, 29.4 %, and 25.3 %, respectively. Selecting different temperatures of spray water and adjusting the evaporator's inlet spray flow rate can improve the refrigerant evaporation temperature and heat pump heating stability.

Modified efficiency-NTU method (m-ε-NTU) for calculating air coolers in dehumidifying or frost conditions. Part IV

In the fourth part of the article, the algorithm for applying the newly developed the m-ε-NTU method for calculating air coolers in dehumidifying or frost conditions is described step by step, and its comparison with the method of segmented division of the heat exchanger is also given. This comparison showed good convergence of the calculation results with a multiple reduction in their execution time. The value of the deviation of the calculated value of thermal power calculated using the newly developed method from the same value calculated using the method of segmented division averaged 3.23% modulo and does not exceed 4.5% modulo. When the heat exchanger is divided into 40 segments, the execution time of the calculation programs increases approximately 18 times compared to using the newly developed method, which can be called a significant advantage of the latter. Considering the above, the newly developed method can be widely used for the selection of air coolers, their verification and design calculations.
 BACKGROUND: A universal method for calculating air coolers that is applicable to design and verification calculations is necessary. The method should consider the effect of dehumidification and frosting on the heat exchange process and allow the quick performance of many calculations to simulate the operation of refrigeration and air conditioning systems without significant loss of accuracy. However, this example of calculation method that addresses all the above-mentioned criteria is unavailable in the domestic and foreign literature.
 AIM: This study develops a universal method for calculating air coolers that is applicable to design and verification calculations. This method considers the influence of dehumidification and frosting on the heat exchange process and allows the quick performance of many calculations to simulate the operation of refrigeration and air conditioning systems without significant loss of accuracy.
 MATERIALS: The developed method for calculating air coolers is based on the classical approach of ε-NTU (efficiency– the number of heat transfer units) and is its adaptation, allowing consideration of the influence of dehumidification and frosting on the heat exchange process and performing the calculation (including the combined operating mode of the air cooler) without dividing the heat exchanger into separate segments. The estimation of the error of calculations performed using the developed method was conducted by comparing the calculated values of the thermal power of the device with those using the segmented division method for various operating modes (including combined).
 RESULTS: Comparison with the segmented division method of the heat exchanger showed good convergence of the calculation results with multiple reductions in execution time. The deviation value of the calculated value of the thermal power computed using the developed method from that using the segmented division method averaged 3.23% modulo and did not exceed 4.5% modulo. When the heat exchanger was divided into 40 segments, the execution time of the calculation programs increased approximately 18 times compared to using the developed method, which can be called a significant advantage of the latter.
 CONCLUSION: The division of the heat exchanger into segments for calculation does not result in a significant increase in accuracy compared with the new method. Thus, the developed m-ε-NTU method can be widely used for the selection of air coolers, their verification, and design calculations.

The Effect of Heat Flux on the Frequency of Bubble Appearance in a Boiling Pool

This research was conducted to determine the effect of heat flux on the frequency of bubbles appearing in boiling ponds. All fluid movement in pool boiling is caused by natural convection currents. The boiling pool consists of four areas of the pool boiling regime. The division of the four areas is based on the value of the heat flux and the difference between the surface temperature of the heater and the fluid. Using a two-phase heat transfer unit (H654 P.A. Hilton machine), The results showed that the power used greatly influences the boiling process, and besides that, the volume of water used also affects the duration of the boiling process. Based on tests using various power levels of 75 W, 110 W, 168 W, 237.5 W, and 290 W. The occurrence of bubbles will be faster and more numerous when using a lower volume of water and greater power. The heat transfer will be greater if a bubble appears, where latent heat plays a very important role. With mathematical analysis, an increase of 1 bubble per minute occurs for every increase in heat flux of 1.3 W/m2.

Theoretical and experimental study on a novel evaluation criteria of heat exchangers applied in aero engines

For the application of heat exchangers in the aeronautic field, an effective evaluation method is required to support the satisfaction of the multi-aspect design restrictions including the thermal and hydraulic performance, occupied volume, total weight, etc. To weigh the benefit and cost for both heat transfer units and heat exchangers, this study thus proposes three evaluation indexes Rp, RV, and Rm defined by the product of heat transfer coefficient and heat transfer area of unit windward area times pressure drop (hA/(AfrontΔp)), that of unit volume (hA/V) and that of unit weight (hA/m). Considering the factors h and A comprehensively and excluding the influence of mean differential temperature, the three indexes can reflect the low flow resistance, compactness, and lightness characteristics under fixed operating conditions and heat transfer duty requirement accordingly. A validation experiment is carried out and the deviation between the evaluation and experiment results is less than 10.88% for the three indexes. To show the applicability, a detailed evaluation of six different heat transfer units and five types of heat exchangers is conducted. A heat transfer unit selection method based on the new indexes is provided finally for heat exchanger design.

Vėdinimo sistemos šilumos perdavimo reguliavimo mazgo termoizoliacijos termografinė analizė

The article analyses the thermal insulation characteristics of the recuperator heat transfer unit of the ventilation system of the public building. The results of the thermographic study showed the temperature distribution on the surface of the system elements. Heat losses are calculated by comparing the temperature of the "hottest" places with the temperature of the "coldest" places in the environment. The research was carried out in late autumn when the outdoor temperature fluctuated between +4 and −2 degrees. It is likely that the heat loss would be significantly higher during the coldest five-day period of winter.

Enclosed thermal management method for high-power photovoltaic inverters based on heat pipe heat sink

Photovoltaic (PV) inverter plays a crucial role in PV power generation. For high-power PV inverter, its heat loss accounts for about 2% of the total power. If the large amount of heat generated during the operation of the inverter is not dissipated in time, excessive temperature rise will reduce the safety of the devices. This paper proposes a closed PV inverter structure based on heat pipe and liquid cooling which overcomes the noise, dust and other problems caused by traditional air-cooling heat dissipation method and reduces cost of the volume occupied inside the body. Heat is dissipated through heat pipes, which are efficient heat transfer units. A simulation model of the actual cabinet was established using computational fluid dynamics (CFD), and the maximum junction temperature in the inverter was investigated under different coolant temperatures, flow rates, cooling liquid and heat loads. The results showed that the liquid cooling heat dissipation structure can effectively dissipate the heat inside the cabinet. The impact of two different types of heat sink used for power modules on temperature uniformity was studied. The results indicated that the 9-heat pipe type heat sink has better heat dissipation and uniform hot spots performance, the maximum heat source temperatures in the chip and capacitor were reduced by 9.91?C and 7.49?C respectively. Finally, the performance of the two types of radiators under different heat loads was studied.

Prediction of critical heat flux and position in narrow rectangular channels using deep feed-forward neural networks coupling with empirical correlations

The critical heat flux (CHF) is a crucial thermo-hydraulic parameter or phenomenon in boiling heat transfer in many industry applications. For a long narrow rectangular channel, a fundamental heat transfer unit in the nuclear industry, the CHF is hard to measure and predict owing to the large aspect ratio (>1000) in geometry. Current technologies can only predict the value of CHF. Still, they cannot predict where the CHF occurs (the positions occurring dry out in narrow rectangular channels) in the large aspect ratio narrow rectangular channel. In this study, we establish a nonlinear relationship between CHF values and their specific occurrence locations under various operating conditions (various inlet mass flow rate, inlet temperature, wall heat flux density) using deep feed-forward neural networks (DFNN) coupled with CHF empirical correlations. Compared with the traditional empirical correlation method and the neural networks (NN) method, the new proposed method can predict both the value of CHF and the specific location where the CHF occurs. By inducing the empirical correlations into the neural networks, the prediction error decreases from 10% to 5% compared to the traditional NN methods. This work aims to provide a predictable and controllable strategy for CHF in narrow rectangular channels.

Numerical Study of the Influence of Different Bending Shapes on the Heat Transfer Characteristics of Annular Cross Wavy Primary Surface Recuperator (CW-PSR)

The cross-wave primary surface recuperator (CW-PSR) is a dependable option as a recuperator for micro gas turbines (MGT). The micro CW-PSR studied in this paper is composed of 171 stacked curved plates, with each plate containing 33 micro heat transfer channels with equivalent diameters of less than 1 mm. In this study, the influence of bending curvature on the thermal performance of CW-PSR plates is investigated through three-dimensional numerical simulation with fluid–solid–thermal coupling. The results indicate that the variation in bending curvature studied can result in a noteworthy 8% difference in the total heat transfer coefficient of CW-PSR plates. A direct correlation between heat transfer capacity and secondary flow strength is derived mathematically, explaining the mechanism by which secondary flow enhances heat transfer. By employing this relationship, a comprehensive analysis of CW-PSR plates with diverse bending curvatures is conducted, effectively showcasing how curvature influences the secondary flow pattern and enhances the channel’s heat transfer capacity. In addition, this paper considers the comprehensive influence of the size parameters of the heat transfer unit and the bending curvature of the heat transfer plate on the heat transfer and flow characteristics of the CW-PSR, and a dominant mathematical expression is obtained, which can be used for the design of similar heat exchangers of the same type.

High Compactness Heat Exchanger Manufactured Using Stereolithography Technology and Photosensitive Resin

Heat exchangers have traditionally been produced on mass using metal alloys and complex manufacturing processes. This work proposes an alternative production via for the additive manufacturing of a cost-effective air-to-air heat exchanger, based on the use of the stereolithography technology. The element has been produced on a FormLabs Form3 printer using standard photosensitive resin. The dimensions of the heat exchanger were 100 × 100 × 100 mm3 and the wall thickness was 0.5 mm. The manufacturing cost of the element was 53.11 €. The heat exchanger was experimentally tested in an air handling laboratory under different climatic conditions. The thermal power of the equipment was 200 W, which is equivalent to a power-volume ratio equal to 200 kW/m3. The experimental energy efficiency was equal to 0.54 (for a number of heat transfer units equal to 1.4) and an overall energy transfer coefficient (U) equal to 1823 W/m2K. In addition, the results showed that the thermal conductivity of the material was less influential the smaller the thickness of the heat exchanger channels. The obtained results show that stereolithography is an economical alternative to obtain customized and high compactness heat exchangers, on demand and just in time.