Identical Pressure Drop Research Articles

Design and Optimization of Cross-Corrugated Triangular Ducts with Trapezoidal Baffles Based on Response Surface Methodology and CFD

Plate heat exchangers are widely used in the Heating, Ventilation, and Air Conditioning (HVAC) field. Cross-corrugated triangular ducts are commonly employed in plate heat exchangers. Inserting baffles into the cross-corrugated triangular ducts can improve the heat transfer performance of the plate heat exchangers. This study focuses on intricate interdependencies among the flow channel apex angle, the trapezoidal baffle inclination angle, baffle position, and Reynolds number (Re) on heat transfer and pressure drop using response surface methodology (RSM) and computational fluid dynamic (CFD). To identify the factors that maximize the Nusselt number (Nu) and minimize friction factor (f), the RSM is used to design factors, conduct numerical studies, and establish regression equations. The results show that the apex angle, baffle angle, X-direction position, and Re have significantly affected Nu and f. Compared to a non-baffled channel with the same apex angle and Re conditions, the optimized channel enhances heat transfer by 1.54 times and has an almost identical pressure drop. The inclined baffle significantly enhances comprehensive performance at low Re. The synergistic effect of the heat transfer and pressure drop is most optimal when the apex angle of the flow channel is 90°, the trapezoidal baffle inclination angle is 52.5°, and the Re is 1000, with the baffle position at 0.625H in the X-direction.

Performance evaluation criteria based on the first and second laws of thermodynamics

Numerical simulations were conducted to investigate the correlation between assessments based on the total entropy generation rate criterion and the thermal resistance criterion in the context of evaluating heat transfer performance. This study focused on a parallel plate channel heat sink with the lower wall subjected to a constant heat flux, under various flow constraints. The flow was assumed to be laminar and fully developed. The energy equation was discretized using the control volume-based power law scheme, and the resulting algebraic equations were solved by the line-by-line method. Optimal channel heights which lead to minimize irreversibility and temperature difference between heated wall and inlet fluid temperatures were determined by both the total entropy generation rate and thermal resistance criteria under four distinct flow constraints: identical mass flow rate, identical inlet velocity, identical pumping power, and identical pressure drop. The investigation revealed a lack of significant correlation between the optimal channel heights derived from assessments based on the total entropy generation rate and thermal resistance. Consequently, the utilization of the total entropy generation rate criterion for evaluating heat transfer performance, based in the second law of thermodynamics, instead of the thermal resistance criterion, based in the first law of thermodynamics, was deemed inappropriate. The total entropy generation rate criterion was based on the minimization of the irreversibility and served as an indicator of heat transfer quality. As a recommendation, it is suggested that results obtained from both the total entropy generation rate and thermal resistance criteria for assessing heat transfer performance be presented separately for each specific application.

Experimental and numerical study of the thermal and hydraulic performance of fin array with wedge shield in bypass

In this study, the thermal and hydraulic performances of fin array with a wedge shield installed in the bypass to alter the flow were experimentally and numerically investigated. The effect of the wedge shield inclination angle was explored at Reynolds numbers ranging from 2753 to 20,825. The mean Nusselt number and friction factor were measured in a wind tunnel with the varying volume rate of inlet air flow. The comprehensive performance evaluation criteria under identical pressure drop and identical pumping power were adopted to evaluate the effect of the wedge shield. The velocity, temperature and pressure fields, and the local Nusselt number were characterized using ANSYS Fluent. The results indicated that the heat transfer enhanced and the flow resistance increased by the wedge shield effect. At a same Reynolds number, the mean Nusselt number and friction factor reach their maximum values at θS=10∘ and θS=60∘, respectively. According to the comprehensive performance evaluation, the applicable condition of wedge shield to enhance fin array performance was the inclination angle θS=135∘ at Reynolds number Re=3286, in which the enhancement was approximately 9% under identical pressure drop and 14% under identical pumping power. The numerical results showed that the velocity increase in fin channel and recirculation zone, caused by the blockage of the wedge shield, was the fundamental mechanism of wedge shield to enhance the heat transfer and increase the flow resistance. The velocity increase and recirculation zone, along with the low temperature region and high pressure in the entrance region of the fin channel caused by them, were all affected by the wedge shield inclination angle.

Three-dimensional shape optimization of fins in a printed circuit recuperator using S-CO2 as the heat-transfer fluid

Printed circuit heat exchangers (PCHEs) are among the most promising candidates for recuperators in the S-CO2 Brayton cycle owing to their high-pressure resistance and high compactness, where the fins are usually installed in channels to improve the thermal-hydraulic performance. The fin shape optimizations have been performed in several previous studies for enhancing the heat transfer and reducing the pressure drop. However, most of them are limited to two-dimensional optimization where the shape variation along the height direction was ignored. In the present study, three-dimensional fin shape optimizations are performed for the PCHE recuperators using S-CO2 as heat transfer fluid to further improve the thermal-hydraulic performance. The heat transfer and pressure drop characteristics in channels with optimized fins are comprehensively evaluated by comparing against conventional cylindrical fins and airfoil fins. The results indicate that the optimized three-dimensional fins feature variable cross-sections along the height direction. Such three-dimensional fins can significantly diminish the impact area of fluid, which is conducive to reducing the local flow resistance caused by high-velocity gradient and consequently improving the comprehensive heat transfer performance. Under the typical working condition studied in this paper, the heat transfer capacity in hot channels of recuperators with the optimized three-dimensional fins is 21.2% greater than the cylindrical fins, and 17.3% greater than the airfoil fins when the identical pressure drop is fixed; while the heat transfer capacity in cold channels with the optimized three-dimensional fins is 24.3% greater than the cylindrical fins, and 29.3% greater than the airfoil fins. In addition, the robustness analysis indicates that the optimized three-dimensional fins can yield more excellent thermal-hydraulic performance than conventional cylindrical fins and airfoil fins over a wide range of working conditions.

The effect of the initial swirl of the supersonic flow of humid air on the adiabatic wall temperature

The paper presents the results of measuring the adiabatic wall temperature of an axisymmetric channel during acceleration of the moist air flow in it to supersonic speed. The initial swirl was imparted to the flow (swirling parameter S = 0.5, 1.0, 2.5). The relative initial humidity (RH) of the flow varied in the range of 10 ÷ 90%. When the flow was accelerated to supersonic speeds, part of the moisture condensed, which influenced the wall temperature. It is shown experimentally that with an increase in the initial moisture content of the flow to certain values, the distribution of the wall temperature for a flow without initial swirl (S = 0) and with swirl with S = 0.5, 1.0 practically coincide. However, from a certain value of RH, the wall temperature in the case of a swirling flow decreases in comparison with a flow without swirling. The maximum decrease in the wall temperature was achieved at RH = 90%. An increase in the initial swirl to S = 2.5 led to a greater decrease in the wall temperature, while the mass air flow through the channel decreased by 26% at an identical pressure drop.

Investigation on Performance Evaluation of Corrugated Low Finned Tubes Oriented for Energy Saving

Heat exchangers are the energy facilities that exchange heat in process industries.Corrugated low finned tube is a kind of helically corrugated tube with trapezoidal shape groove. In this paper, the comparison approach is adopted to evaluate the overall performance of corrugated low finned tubes. The heat transfer and friction factor correlations were firstly introduced. Then the overall performance was evaluated for the identical pressure drop and identical pumping power. Finally, the performance evaluation criteria were fitted as the function of Reynolds number, Prandtl number, pitch to diameter ratio and rib height to diameter ratio. The results showed thatboth the evaluation factors at identical pumping power and at the identical pressure drop are greater than 1.0 over the entire Reynolds number for all the enhanced tubes, indicating that the enhanced tubes have the second best performance oriented from energy-saving.

Thermal-hydraulic performance enhancement analysis of microtube with superhydrophobic surfaces

Microchannels have an excellent thermal performance but they acquire a large pressure drop. Recent studies show that microchannels with superhydrophobic surfaces may provide an innovative approach to achieve a balance between thermal and hydraulic performances. In this work, the fluid flow and heat transfer in microtube with superhydrophobic surfaces are analyzed at fully developed laminar flow, and the formulas of fRe and Nu are obtained. Comparisons of thermal resistances on superhydrophobic and smooth microtubes are performed at identical flow rate, pressure drop and pumping power, respectively. At indentical flow rate, superhydrophobic surface can’t enhance the thermal-hydraulic performance. At identical pressure drop or pumping power, the critical pressure drop and pumping power exist respertively, which is the boundary point of the thermal-hydraulic performance between superhydrophobic and smooth microtubes. The critical parameters are deduced to help determining whether superhydrophobic surfaces enhance thermal-hydraulic performance.

Design and optimization of a novel high temperature heat exchanger for waste heat cascade recovery from exhaust flue gases

The waste heat of high temperature exhaust flue gases is widely distributed in many industrial processes. Recovery of waste heat is of great significance to energy saving and sustainability. In this paper, a novel high temperature heat exchanger with hybrid enhancement technologies is proposed to improve waste heat recovery efficiency based on the cascade recovery and utilization method. Algorithm for HTHE structural design and optimization is developed and verified according to the experimental results. Heat transfer and pressure drop performance of the proposed HTHE are estimated by using the algorithm. The results show that the effectiveness of the proposed HTHE increases as the gas temperature increases and mass flow rate decreases. Average effectiveness of the proposed HTHE and temperature of preheated air are 12.5% and 85.8 °C higher than those of traditional HTHE with additional 70.0% and 22.0% pressure drop on air and gas sides, respectively. The structural optimization of the proposed HTHE is carried out and it shows that the optimized HTHE has better heat transfer capacity and comprehensive performance under identical pressure drop, increasing effectiveness by 12.6% without enlarging pressure drop compared with the non-optimized HTHE.

Thermo-Hydraulic Performance Evaluation, Field Synergy, and Entransy Dissipation Analysis for Hexagon-Like and Circular-Like Pin Finned Tube Bundles

ABSTRACTIn this paper, a comprehensive thermo-hydraulic performance evaluation for air flow across the hexagon-like and circular-like staggered pin finned tube bundle heat transfer surfaces has been numerically carried out by adopting the performance evaluation plot of enhanced heat transfer techniques oriented for energy-saving. In addition, the simulation results have also been analyzed from the viewpoints of field synergy principle and entransy dissipation extreme principle. The results indicate that the heat transfers are all enhanced based on identical pressure drop for the hexagon-like and circular-like pin finned tube bundles within the inlet velocity range from 1 m/s to 10 m/s studied. Moreover, the circular-like pin finned tube bundle offers the lowest friction factor increase ratio for the same Nusselt number increase ratio. Furthermore, the synergy between velocity and fluid temperature gradient has been proved again, having inherent consistency with the dissipation of entransy.

Investigation on turbulent flow and heat transfer characteristics and technical economy of corrugated tube

In this work, experimental and numerical study was performed to determine the heat transfer and pressure drop characteristics of corrugated tubes. Measurement and simulation are conducted for three kinds of corrugated tubes. The corresponding plain tubes were simulated at the same time. In the numerical simulation the flow is modeled as being axisymmetric and fully elliptic by using body-fitted finite volume method and the Low-Rek-ε turbulent model. The uniform heat flux boundary condition is specified to simulate the electrical heat using in the experiments. The heat transfer performance of the corrugated tubes is compared with plain tubes under three constraints (identical mass flow rate, identical pumping power and identical pressure drop). The experimental results and numerical results are in good agreement. It was found that the corrugated tubes have a good synergy than plain tubes and an appreciable increase in heat transfer performance. The corrugated tube possesses a large market potential.

Analytical solutions of force convective heat transfer in plate heat exchangers partially filled with metal foams

This paper investigated the flow and thermal characterization of parallel-plate heat exchangers partially filled with metal foams analytically. Firstly, the velocity and temperature distributions in partially filled symmetrical channel have been plotted and the effect of key parameters (e.g. porosity, pore density, height of metal foams, flow ratio in metal-foam filled region) on its flow and heat transfer performance have been analyzed. Then, the results have been applied for combined heat exchangers with symmetrical and asymmetrical channels to have optimized heat transfer performance and relatively low and identical pressure drop for each channel.

Fuel filtration properties and mechanism of a novel fibrous filter produced by a melt-process

A porous filter comprised of polypropylene (PP)/polyamide 6 (PA6) micro-/nano-fibers has been produced from a novel melt-based plastics co-extrusion and exfoliation process technology for the separation of microscale water droplets from diesel fuel. The filter media have a mean pore size of 11µm, a porosity greater than 85%, a surface area of more than 12x the value for an electrospun medium, tough mechanical properties, and a contact angle of 124°, all of which make the co-extruded filter a superior fuel filter. An improvement in water-separation from diesel fuel feed stream from 62–85% water-removal was achieved by the co-extruded and exfoliated PP/PA6 fibrous media in comparison to a commercial fuel filter under identical test conditions and pressure drop. The fuel/water separation mechanism was identified as a capture-coalescence-release process. It has been found that the highest performance fuel filter required small pore sizes and large surface area for droplet capture, due to the hydrophobic nature of materials required for successful droplet release, and a high porosity for minimizing flow resistance. In addition to the proof of concept water separation demonstration, studies into (a) the effect of increasing the barrier substrates thickness and (b) surface corona treatment to alter the hydrophilicity of media surfaces on the filtration efficiency co-extruded, exfoliated filters are reported.

CFD analysis of pressure drop across grid spacers in rod bundles compared to correlations and heavy liquid metal experimental data

Early studies of the flow in rod bundles with spacer grids suggest that the pressure drop can be decomposed in contributions due to flow area variations by spacer grids and frictional losses along the rods. For these shape and frictional losses simple correlations based on theoretical and experimental data have been proposed. In the OECD benchmark study LACANES it was observed that correlations could well describe the flow behavior of the heavy liquid metal loop including a rod bundle with the exception of the core region, where different experts chose different pressure-loss correlations for the losses due to spacer grids. Here, RANS–CFD simulations provided very good data compared to the experimental data. It was observed that the most commonly applied Rehme correlation underestimated the shape losses. The available correlations relate the pressure drop across a grid spacer to the relative plugging of the spacer i.e. solidity emax. More sophisticated correlations distinct between spacer grids with round or sharp leading edge shape. The purpose of this study is to (i) show that CFD is suitable to predict pressure drop across spacer grids and (ii) to access the generality of pressure drop correlations. By verification and validation of CFD results against experimental data obtained in KALLA we show (i). The generality (ii) is challenged by considering three cases which yield identical pressure drop in the correlations. First we test the effect of surface roughness, a parameter not present in the correlations. Here we compare a simulation assuming a typical surface roughness representing the experimental situation to a perfectly smooth spacer surface. Second we reverse the flow direction for the spacer grid employed in the experiments which is asymmetric. The flow direction reversal is chosen for convenience, since an asymmetric spacer grid with given blockage ratio, may result in different flow situations depending on flow direction. Obviously blockage develops differently along flow direction due to the asymmetry, so that the same numerical mesh can be used for two distinct simulations. The study shows that prediction of pressure drop across spacer grids by RANS–CFD using well resolved meshes in the spacer grid region yields more accurate results than any available correlation.

Heat Transfer and Frictions in the Rectangular Divergent Channel with Ribs on One Wall

An investigation of ribbed divergent channel was undertaken to determine the effect of rib pitch to height ratio on total friction factor and heat transfer results in the fully developed regime. The ribbed divergent rectangular channel with the channel exit hydraulic diameter (D ho ) to inlet channel hydraulic diameter (D hi ) ratio of 1.16 with wall inclination angle of 0.72 deg, at which the ratios (p/e) of 6,10, and 14 are considered. The ribbed straight channel of D ho /D hi =1.0 were also used. The ribbed divergent wall is manufactured with a fixed rib height (e) of 10 mm and the ratio of rib spacing (p) to height 6, 10, and 14. The measurement was run with range of Reynolds numbers from 24,000 to 84,000. The comparison shows that the ratio of p/e=6 has the greatest thermal performance in the divergent channel under two constraints; identical mass flow rate and identical pressure drop.

Numerical simulation analysis and optimum design for combined type pressure reducing valves

Pressure reducing valve is an extremely significant equipment of energy dissipation for the water supply by gravity with pressure reducing technology in hydropower stations, and which has a pronounced effect on the normal technical water supply even safety operation for the hydropower units. A three-dimensional numerical calculation of flow field and cavitation characteristics towards a combined type pressure reducing valves was carried out based on the system of technical water supply in this paper. The numerical results show that the investigated valve could meet the requirements of technological supply water pressure and great pressure loss was caused when the water flow was accelerated by narrow overflowing section between throttling cone and valve seat. At working operation, obvious cavitation phenomenon was observed on the surface of throttling cone, and the maximum volume fraction of vapor reached 0.537%. Based on above researches, this paper introduces an optimization model for profile line design of throttling cone. The optimal results show that the cavitation performance is effectively improved with identical pressure drop compared with original results.

Effects of the arrangement of triangle-winglet-pair vortex generators on heat transfer performance of the shell side of a double-pipe heat exchanger enhanced by helical fins

To improve heat transfer performance of the shell side of a double-pipe heat exchanger enhanced by helical fins, triangle-winglet-pair vortex generators (VG) were installed along the centerline of the helical channel with rectangular cross section. The effects of the arrangement of the triangle-winglet-pair VG, such as the geometry, the angle of attack and the quantity on heat transfer performance and pressure drop characteristics have been investigated experimentally to find out the optimal design of the VG. Air was used as working fluid within the range of Re from 680 to 16,000. The results show that, the heat exchange effectiveness of the shell side with VG is 16.6 % higher than that without VG. The vortices and the unsteadiness of the flow introduced by the VG make a great contribution to the increase. Under identical pressure drop condition, the angle of attack of 30° is the best choice compared with 45° and 60°. Under the three constraints, i.e., identical mass flow rate, identical pressure drop and identical pumping power, the largest VG size can achieve the best enhancement effect. Installation of three pairs of VG within one pitch is an optimal design for the shell side used in the present experiments. The enhancement effect of isosceles right triangle is better than that of right triangle in which one acute angle is 30°.

Convective Heat-Transfer Characteristics of a Channel With One Surface Having Mini-Grooves in the Flow Direction and a Plain Surface Located at a Mini-Distance

Insulated-gate bipolar transistors (IGBTs) have been widely used in power transmission systems in recent years. However, a significant amount of electric energy is converted into heat when they are operated. It is necessary to maintain the temperature of the transistor junction in the power component below the maximum allowable temperature for its normal operation. In this paper, the convective heat-transfer characteristics of a channel with one surface having mini-grooves in the flow direction and another plain surface located at a mini-distance are reported. The surfaces can be arranged as a sandwiched-type cooler, in which the core plate with plain surfaces is located between two cold plates, with one surface having mini-grooves. Two IGBT units are mounted on the outside wall of the cold plates to save space. Water and a mixture of water and ethylene glycol are used as the coolants. The heat-transfer characteristics of the cooler are experimentally investigated. For comparison, a cooler with the same-sized plain-surface channels is tested under the same experimental conditions. The correlations of the mean Nusselt number and the friction factor are obtained, and the heat-transfer enhancement performance is evaluated under the identical mass flow rate, the identical pressure drop, and the identical pumping power criteria. The heat-transfer performance of the surface studied is much better than that of the microchannel surface under conditions in which a large area needs to be cooled. The results show that the mini-grooved channels can effectively enhance the heat transfer.

Effects of angled ribs on turbulent heat transfer and friction factors in a rectangular divergent channel

Heat transfer and friction factors of fully developed turbulent flows in the stationary rectangular divergent channel with parallel angled ribs have been investigated experimentally. Four different parallel angled ribs (α = 30°, 45°, 60°, and 90°) are placed to the channel's two opposite walls as well as to the channel's one sided wall only, respectively. The ribbed rectangular divergent channel has the inclination angle of 0.72° at the left and right walls, corresponding to Dho/Dhi = 1.16. The divergent channel has the cross sections of 100 × 75 mm2 at inlet and 100 × 100 mm2 at exit. The ribbed walls are manufactured with a fixed rib height (e) = 10 mm and the ratio of rib spacing (p) to height (e) = 10. The measurement was conducted within the range of Reynolds numbers from 15,000 to 89,000. Thermal performances of the ribbed rectangular divergent channel with different flow attack angles of ribs are compared with the smooth straight circular channel under three constraints: identical mass flow rate, identical pumping power, and identical pressure drop.

Performance comparison of heat exchangers with different circuitry arrangements for active chilled beam applications

Abstract This paper presents a performance comparison study of heat exchangers with different circuitry arrangements for active chilled beam (ACB) applications. Supported by a two-way discharge ACB terminal unit and a performance test pilot plant, four 2-rows plain fin and tube heat exchangers, namely, 1-circuit, 2-circuits, 4-circuits, and 8-circuits, are investigated under a wide range of chilled water volume flow rates. The chilled water side thermal and hydraulic characteristics are evaluated. The heat transfer rates are compared under three sets of criteria: identical water volume flow rate, identical pressure drop, and identical pumping energy consumption. Meanwhile, the comparisons are also performed in terms of two non-dimensional parameters: heat exchanger effectiveness and performance index. The study shows that different circuitry arrangements should be preferred in different operation conditions and under different evaluation criteria, while the 2-circuits arrangement is the most comprehensive and reasonable option rather than the 1-circuit one that is widely used in all available ACB products.

Thermal Performances in Ribbed Rectangular Convergent and Divergent Channels

The rectangular convergent/divergent channels with one sided ribbed surface only have the inclination angles of 0.72oand1.43o at which the ribbed wall is manufactured with a fixed rib height ( e) =10 mm and the ratio of rib spacing (p) to height ( e) =10. The comparison shows that among the four channels (Dho/Dhi =0.67, 0.86, 1.16, and1.49) the divergent channel of Dho/Dhi =1.49 has the highest thermal performance at the identical mass flow rate, and the divergent channel of Dho/Dhii =1.16 has the highest at the identical pumping power and static pressure drop.