Friction Factor Correlations Research Articles

Air side performance characterization of wavy Fin-and-tube heat exchangers having elliptic tubes with large waffle heights

Demand of less energy consumption and higher energy efficiency suggests increasing heat transfer performance of a fin-and-tube heat exchanger. This paper experimentally and numerically examines the air side performance of wavy fin-and-tube heat exchangers (FTHXs) with elliptic tubes. In a properly regulated and evenly distributed airflow supply loop, the inlet/outlet temperatures and pressure drops across the test section are obtained using two measuring meshes and a differential pressure transducer to resolve thermal and frictional outcomes in terms of the heat transfer coefficient and pressure drop. The theoretical formulation considers the thermo-fluid analysis of crossing airflows over the wavy finned tube heat exchanger. In this research, the test heat exchanger contains elliptic tubes incorporated with 4-row wavy fins, while the proposed design adopts a large setting of 3.24-mm waffle height and 3.02-mm fin pitch, respectively. The predicted heat transfer coefficients and pressure drops are in good agreement with the measured data at varied inlet air velocities to validate the CFD model. Numerical simulations are then extended to investigate the influences of waffle height, fin pitch, wave length and inlet air velocity on thermofluid characteristics of FTHXs. Six common correlations of Colburn factors (j) and friction factor (f) factors are compared with predictions to evaluate their applicabilities for reasonable estimates of thermal and frictional performance of FTHXs. It is noticed that the correlation by Wang et al. can achieve the most accurate results in calculating the j and f factors.

Computational fluid dynamics simulations to improve performance characteristics of a manifold having a central inlet and outlet

In the present work, performance/flow characteristics (namely, the effect of operating parameters like pressure on flow patterns, pressure drop, and the extent of flow uniformity) and transport phenomena of a manifold (header tube assembly) having an inlet and outlet at the center are carried out on a macroscale geometry using CFD simulations. In this study, an existing design available in the published literature (with high flow non-uniformity) was considered and an optimized design (with minimum flow non-uniformity) was developed. The optimization is performed by incorporating a perforated plate (distributor) inside the top header of the manifold. First, CFD simulations for different configurations of the existing design with the perforated plate have been performed for a pressure of 10 bar with steam as a working fluid, and an optimized configuration having a minimum flow non-uniformity of less than 3% is obtained. CFD simulations for both the existing design and optimized design are then performed for a pressure range (10 ≤ p ≤ 70 bar) and the corresponding Reynolds number (Re) range (2.82E+05 ≤ Re ≤ 2.82E+06) with steam as the working fluid. The extent of non-uniformity (ENU) and pressure drop for the existing design (without a distributor) and optimized design (with a distributor) have been analyzed and compared. The optimized design gives the near uniform flow (∼1–4%) for all pressures and Reynolds numbers considered. An empirical correlation relating the friction factor (as per the Chilton–Colburn analogy) and Re has been developed for both designs (with and without a distributor). The predicted friction factors are compared with the present CFD predictions, and experimental data of the shell and tube heat exchanger are available in the published literature. A good agreement within a 10–15% deviation has been observed. Based on the Chilton–Colburn analogy, a correlation for the Nusselt number is obtained from the friction factor correlations for both with and without distributor cases. The correlations for friction factors were found to be valid under any operating conditions for a pressure drop range within 0.05 < ∆p < 1.8 bar irrespective of the design of the distributor, assuming that the manifold is able to withstand the pressure drops in the given range.

Analysis and evaluation of friction factor correlations for the determination of pressure drop through packed adsorption columns

The application of different pressure drop correlations for packed bed columns was evaluated in a solution algorithm for the dynamic adsorption process model. There were found differences in bed saturation time for the ethanol-water separation study case. Computational time required also shown significant differences between the correlations evaluated.

Flow maldistribution and heat transfer characteristics in plate and shell heat exchangers

• The pressure drop, flow maldistribution, and heat transfer rate are measured in a plate and shell heat exchanger. • Friction factor and Nusselt number correlations are provided for water and oil flows. • The plate side showed a better thermal performance than the shell side but a higher friction factor. • Flow maldistribution was more severe on the shell side than on the plate side. • The effectiveness deterioration due to the flow maldistribution was less than 4%. • An analytical comparison indicates that PHEs have a better thermo-hydraulic performance than the PSHE studied. The plate and shell heat exchanger (PSHE) was developed to overcome the traditional gasketed plate heat exchangers (PHE) operation limits. Its constructive characteristics allow higher-pressure and thermal applications, suitable for processes found in power plants and the oil and gas industry. However, studies on the PSHE thermo-hydraulic performance are still scarce. This study presents an experimental and theoretical analysis of the flow and heat transfer characteristics of a PSHE. A test rig operates with water and viscous oil to produce turbulent and laminar flow regimes, typical of the oil and gas industry. The heat transfer rate, pressure drop, and flow distribution are measured. The m 2 -model, developed to predict flow maldistribution on the PHE, is suitable for the plate side of the PSHE. An analytical model is used to correct the effects of maldistribution on the overall heat transfer coefficient and provide Nusselt number correlations. Experiments show that the flow maldistribution increases the heat exchanger pressure drop and deteriorates the heat transfer performance. The maximum and average channel flow rate ratio reaches 2.30 on the shell side and 1.75 on the plate side. Friction factor correlations are created based on the channel pressure drop data. The shell side has an inferior overall performance than the plate side, with a higher maldistribution and a lower Nusselt number. The PSHE effectiveness deterioration due to the maldistribution is 4% in the worst scenario within the experimental range. Results indicate that the PHE thermo-hydraulic performance is superior to the PSHE. Nevertheless, the structural advantages of the PSHE make it appropriate for applications involving high pressures and elevated temperatures.

Machine-learning-aided design optimization of internal flow channel cross-sections

• Artificial neural network-based models are developed for prediction of nusselt number and friction factor. • The predictions apply for fully developed laminar flow in constant cross-section ducts of different cross section shapes. • The machine learning (ML) model predictions are validated and compared against numerical simulations. • Use of ML models in shape optimization of channel cross section for different objective functions is demonstrated. • Guidelines for usage of the developed ML models are discussed. The design optimization of various thermal management components such as cold plates, heat sinks, and heat exchangers relies on accurate prediction of flow heat transfer and pressure drop. During the iterative design process, the heat transfer and pressure drop is typically either computed numerically or obtained using geometry-specific correlations for Nusselt number and friction factor. Numerical approaches are accurate for evaluation of a single design but become computationally expensive if many design iterations are required (such as during formal optimization processes). Correlation-based approaches restrict the design space to a specific set of geometries for which correlations are available. Surrogate models for the Nusselt number and friction factor, which are more universally applicable to all geometries than traditional correlations, would enable flexible and computationally inexpensive design optimization. The current work develops machine-learning-based surrogate models for predicting the Nusselt number and friction factor under fully developed internal flow in channels of arbitrary cross section and demonstrates use of these models for optimization of the cross-sectional channel shape. The predictive performance and generality of the machine learning surrogate models is first verified on various shapes outside the training dataset, and then the models are used in the design optimization of flow cross sections based on performance metrics that weigh both heat transfer and pressure drop. The optimization process leads to novel shapes outside the training data, and so numerical simulations are carried out on these optimized shapes to compare with the surrogate model predictions and show their performance is at least as good as that of shapes with known correlations available. A three-lobed shape was found to reduce friction factor, whereas a pentagon with rounded corners and an ice cream cone-shaped duct, both found using different performance metrics. Although the ML model predictions lose accuracy outside the training set for these novel shapes, the predictions follow the correct trends with parametric variations of the shape and therefore successfully direct the search toward optimized shapes.

Experimental evaluation on heat/mass transfer and pressure drop of a microchannel membrane‐based desorber for compact and efficient H2O/LiBr absorption refrigeration

• Microchannel membrane-based desorber using H 2 O/LiBr is experimentally studied. • The lower channel performs better in heat/mass transfer but worse in pressure drop. • Nu and Sh increase with Re and inlet temperature but decrease with concentration. • f increases with inlet concentration but decreases with Re and inlet temperature. • Prediction accuracies of Nu, Sh , and f are improved by 38.83%, 62.76%, and 78.18%. Microchannel membrane-based desorber plays a vital role in compact and efficient absorption refrigeration systems to utilize renewable and waste energy. However, its existing models have certain errors because they are based on the empirical correlations that developed for other processes. In this study, experiments are carried out to evaluate the heat/mass transfer and solution pressure drop characteristics of a H 2 O/LiBr microchannel membrane-based desorber (1 mm width, 250 mm length, 0.15 and 1 mm height, and 100 channel numbers) with mass concentration ranging between 50 and 60 wt% and inlet temperature between 65 and 85 °C. The desorber with a lower channel height has a higher overall heat transfer coefficient, a higher desorption rate, and a greater pressure drop. But when the inlet temperature reaches 85 °C, the higher channel yields an unexpected higher desorption rate (0.00645 kg/(m 2 ⋅s)) than the lower channel (0.00615 kg/(m 2 ·s)). The experimental results show that both heat and mass transfer processes are strengthened by a higher solution flow rate, higher inlet temperature, and lower concentration, while the solution pressure drop is significantly reduced by a lower flow rate, lower concentration, and higher inlet temperature. Based on the experimental data, empirical correlations for Nusselt number, Sherwood number, and friction factor are obtained with errors of ± 40%, ± 20%, and ± 25% with prediction accuracies improvement of 38.83%, 62.76% and 78.18% compared to the correlations from the literature, facilitating the evaluation and design of compact and efficient absorption refrigeration systems.

Flow resistance analysis of non-isothermal supercritical CO2

• Supercritical CO 2 turbulent flow in gas-like region was studied. • Friction factor of supercritical CO 2 decreases with the increase of heat flux. • The density and turbulence fluctuation are the key to thermally-induced mechanism. • Proposed dimensionless number can evaluate thermally-induced and inertial forces. • Proposed correlation of supercritical CO 2 friction factor has excellent accuracy. Supercritical carbon dioxide power cycle is considered as one of the promising systems of next generation power cycle. Nevertheless, the hydraulic characteristics of typical heat source components under operating conditions (gas-like region) are not clear, and the present conclusions for flow characteristics of non-isothermal fluids are also not consistent. In this work, the turbulent flow of supercritical CO 2 in a vertical upward tube was studied experimentally and numerically. The range of experiment parameters was set as 750–2800 kg/(m 2 s) of mass flux, 10–28 MPa of pressure, 200–410 kW/m 2 of heat flux and 65–500 °C of bulk flow temperature. Furthermore, the feasibility of SST k-omega model was verified by experiment results. The effects of mass flux and pressure on friction pressure drop under non-isothermal conditions are basically consistent with those under isothermal conditions. However, the increase of heat flux leads to the decrease of friction pressure drop and friction factor. Under high heat flux condition, both the viscous shear stress and Reynolds shear stress decrease. The viscous shear stress contributes slightly to the mechanical energy dissipation, hence the Reynolds shear stress is the crux to the change of friction pressure drop. Further studies show that the decrease of density in near-wall region and the decrease of turbulence fluctuation in core region are critical to the decrease of mechanical energy dissipation. According to the theoretical and dimensional analysis, the dimensionless number Xi that can be qualitatively analyzed for the ratio of thermally-induced force to inertial force was obtained. Finally, a high-precision correlation of friction factor was proposed. The proportion of calculated value within 10% and 30% error ranges is 76.65% and 100%, respectively.

Heat transfer and friction factor correlations development for double pass solar air heater artificially roughened with perforated multi-V ribs

Thermo-hydraulic performance of a Double Pass Parallel Flow Solar Air Heater (DPPFSAH) roughened with perforated multi-V ribs has been studied experimentally in a prior study and the effect of variation in open area ratio and change in relative roughness width has been analysed and reported. The current work builds upon the aforementioned earlier study by discussing in detail the methodology and various steps involved in the development of a correlation for variable parameters with Nusselt number and friction factor for DPPFSAH. The outcomes show that perforations in the multi-V ribs lead to a considerable rise in the Nusselt number, a 9.66 times increase in the thermo-hydraulic performance parameter and nearly a four times increase in friction factor compared to multi-V ribs with smoothed walls. Empirical correlations for the Nusselt number and friction factor were obtained for the double pass parallel flow solar air heater with perforated multi-V ribs established with ±14% and ±7%.

Thermal-hydraulic performance of zigzag channels with Bending number below unity for printed circuit heat exchanger

• A dimensionless number is proposed for characterizing the bending of zigzag channel. • Zigzag channel with Bending number below unity is studied experimentally. • Empirical correlations are obtained for friction factor and Nusselt number. • Thermal-hydraulic performance of the tested zigzag channel is obtained. Zigzag-channel printed circuit heat exchanger is one of the leading candidates for the pre-cooler of supercritical carbon dioxide Brayton cycle. A dimensionless number, Bending number, is newly proposed to characterize the bending amplitude of zigzag channels in printed circuit heat exchanger. The existing studies are examined with the help of Bending number and it is found that the thermal-hydraulic performance of zigzag channels with Bending number below unity has not been investigated. To remedy this situation, the thermal-hydraulic performance of zigzag channels with Bending number 0.49 is tested. Experimental result shows an enhancement in convective heat transfer of supercritical carbon dioxide at its pseudo-critical zone. It is also found that there is no obvious transition from laminar flow to turbulent flow within Reynolds number 1560-45700. Flow and heat transfer correlations for the tested zigzag channels are proposed. The new developed Fanning friction factor correlation predicts almost 100 % of the experimental data within ±20 % error band, and the new developed heat transfer correlation predicts 78.6 % of the experimental data within ±25 % error band. Comparison with Ngo’s correlations, Kim’s correlations, and Saeed’s correlations demonstrates that both Fanning friction factor and Nusselt number are higher with larger Bending number. Analysis of the thermal-hydraulic performance, (Nu/Nu 0 )/(f/f 0 ) 1/3 , of the tested zigzag channels shows that its combined performance is improved obviously compared with straight channels when both Re and Pr are large.

Laminar convective heat transfer in helical twisted multilobe tubes

Augmentation of heat transfer performance has long been the main interest in thermal fluid engineering sector. Some commonly adopted thermal enhancement methods are adoption of helical tubes, twisted tubes and multilobe cross-sections. Several studies reported further enhancement when combination of these methods were implemented. Despite their promising potential, no investigation on the performance of heat transfer in helical twisted multilobe tubes has been reported. Therefore, present study is conducted with the main objective to study the laminar convective heat transfer of Newtonian fluid in a helical twisted multilobe tube through computational fluid dynamics (CFD) simulations. A three-dimensional model was developed with accordance to principles of fundamental conservation. The model was validated against available experimental data for which a good agreement was achieved. The model was thus utilised to investigate the fluid flow and heat transfer in the studied tube within a range of certain parameters, such as, number of lobes, number of twists, inlet Reynolds number as well as the geometry of the pipe (straight and helical). The performances of heat transfer of the investigated configuration are evaluated using the typical Nusselt number, friction factor and performance index (PI), which is a ratio between average Nusselt number and friction factor under constant pumping work condition. The results reveal that adding the number of lobes alone has negligible effect on the performance. However, combination of number of lobes and tube twisting result in considerable changes on the heat transfer coefficient with marginal effect on the friction factor. This is especially pronounced for tube with even number of lobe (bilobe and quadrilobe). Overall, helical coiled tube performs better within the range of Re 500 to Re 2000 as the performance index averagely increase by 18.4% as compared to straight tube, whereas below Re 500, straight tube is slightly outperforming the helical tube (3.65%). Twisting of the tubes result in the opposite effect, i.e. it improves the performance of straight tube but deteriorate the performance of helical tube. Thus, twisting is recommended primarily for straight tubes. Highest performance index yielded is 7.01 by bilobe helical tube without twist at Re 2000. Lastly, correlations for friction factor and Nusselt number are developed for straight and helical multilobe twisted tube within studied range. This correlation can serve as quick design tools for non-conventional heat exchanger utilizing twisting multilobe tubes.

Full-Core Coupled Neutronic, Thermal-Hydraulic, and Thermo-Mechanical Analysis of Low-Enriched Uranium Nuclear Thermal Propulsion Reactors

Nuclear thermal propulsion is an enabling technology for future space missions, such as crew-operated Mars missions. Nuclear thermal propulsion technology provides a performance benefit over chemical propulsion systems by operating with light propellants (e.g., hydrogen) at elevated engine chamber conditions. Therefore, nuclear thermal propulsion reactor cores exhibit high propellant velocities and elevated propellant and fuel temperatures, subsequently leading to relatively high thermal stresses and geometrical deformation. This paper details the numerical approach to solve the thermo-elastic equations, which was implemented into the recently developed ntpThermo code. In addition, this paper demonstrates the extension of the Basilisk multiphysics framework to perform full-core coupled neutronic, thermal-hydraulic, and thermo-mechanical analysis of nuclear thermal propulsion reactors. The analyses demonstrate and quantify thermo-mechanical feedback, which for the investigated cases, acted to reduce maximum fuel temperatures and pressure drop across the fuel element channels. Thermo-mechanical feedback had a significant impact on the mass flow distribution within the reactor core and, thus, a substantial impact on solid-material temperatures and stresses, but only a minor impact on reactivity and local power distributions. Sensitivity studies revealed that the friction factor correlation applied to perform the analysis has a significant impact on the pressure drop across the fuel element channels. The most important observation of this research is the importance of incorporating the thermo-mechanical feedback within an integrated multiphysics solution sequence to enable the consistent design of future nuclear thermal propulsion systems.

Numerical investigation on thermal–hydraulic performance of supercritical LNG in a Zigzag mini-channel of printed circuit heat exchanger

• Flow mechanism of supercritical hydrocarbon fluids was investigated. • Comparison on thermal performances for two hydrocarbon fluids was compared. • Correlations for supercritical hydrocarbon fluids in a mini-channel were proposed. Printed circuit heat exchanger (PCHE) is receiving wide attention as a new kind of compact heat exchanger. It is considered as a promising vaporizer in floating liquefied natural gas (FLNG) vessels due to its compactness and high efficiency. In this paper, the numerical model with two Zigzag mini-channels was built. The cross section was a semicircle with the diameter of 1.5 mm and the overall length was 200 mm. In the simulation, SST k - ω turbulent model was employed and validated against experimental data of supercritical CO 2 flow. The mechanism of flow and heat transfer was investigated, and the influence of operating parameters including mass flux, outlet pressure and inlet temperature was analyzed from the perspective of high-speed components of turbulent core area, degree of flow separation, energy transport and dissipation rate. The results indicated that the worse flow performance and better heat transfer performance exhibited at higher mass flux and inlet temperature conditions, whereas the higher outlet pressure conditions exhibited the better flow and heat transfer performances. Correlations for the Fanning friction factor and Nusselt number were developed within a deviation of ±5%, aiming to apply to the prediction of thermal–hydraulic performance of supercritical LNG.

Experimental Investigation on Heat Transfer and Pressure Drop of Supercritical Carbon Dioxide in a Mini Vertical Upward Flow

Experiments on the convection heat transfer and pressure drop of supercritical carbon dioxide in a mini vertical upward flow were investigated in a smooth tube with an inner diameter of 2 mm. The experiments were conducted with pressures ranging from 7.62 to 8.44 MPa, mass fluxes ranging from 600 to 1600 kg·m−2·s−1, and heat flux ranging from 49.3 to 152.3 kW·m−2. Results show that the peak of heat transfer occurs when the bulk fluid temperature is below the proposed critical temperature and the wall temperature is above the proposed critical temperature. For the 2 mm vertical upward flow, the radial buoyancy effects are relatively weak, and the axial thermal acceleration effect cannot be negligible. In this study, a new modified Jackson correlation for the supercritical carbon dioxide is proposed for convective heat transfer. To reflect the effect of flow acceleration on heat transfer, a dimensionless heat flux was introduced to construct a new semi-correlation of heat transfer. The new correlation of friction factor taking into account the variation of density and dynamic viscosity was proposed with 146 experimental data within a ±20% error band.

Performance Evaluation of Plate Heat Exchanger Using CuO-DI Water Nanofluid

Abstract Experimental and numerical investigations have been carried out for performance evaluation of plate heat exchanger (PHE) using CuO-DI water nanofluid at different volume concentrations of (ϕ = 0, 0.005, 0.01, 0.03, 0.05, and 0.07) for the Reynolds number, Re = 71–1350, Prandtl number, Pr = 3.18–13.7, and at fixed inlet temperatures of hot and cold fluids Thi = 60 °C and Tci = 20 °C, respectively. The two main influential thermo-physical properties of the nanofluid, namely, dynamic viscosity and thermal conductivity obtained for various volume fractions at different temperatures, are further utilized for performance analysis of the PHEs. The thermo-hydraulic performance of PHE is evaluated based on the J.F. factor. According to experimental and simulated results, volume concentration, ϕ = 0.03 is found to be the optimum concentration of nanofluids. The individual correlations for Nusselt number, friction factor, dynamic viscosity, and thermal conductivity in terms of nanofluid volume fraction are proposed based on the experimental results.

Mathematical Modeling of Efficiency Evaluation of Double-Pass Parallel Flow Solar Air Heater

To investigate the influencing range and optimize values of different operational and system parameters on the double-pass parallel flow solar air heater’s (DPPFSAH) thermal, effective, and exergetic efficiencies, an iterative method was used to analyze the governing energy equations using a theoretical model written in MATLAB based on the Nusselt number (Nu) and friction factor (f) correlations developed in the work performed earlier. A comparison between double-pass and single-pass SAHs for mathematical and experimental outcomes was conducted, and the results were found to be fairly consistent. According to the thermo-hydraulic performance indicators, similar to single-pass SAHs, perforated multi-V rib-roughened DPPFSAHs achieve optimum thermal performance for lower Reynolds numbers, which does not change much as the Reynolds number increases above 18,000. This finding can be taken into account when designing any DPPFSAH.

Study on flow and heat transfer of liquid metal in the microchannel heat sink

In order to overcome the shortcoming of high resistance of liquid metal flow, three new types of microchannel heat sinks, namely top-slotted microchannel, bottom-extension microchannel and composite microchannel are proposed, and the flow and heat transfer characteristics of liquid metal galinstan are numerically studied. It is found that all three new microchannel heat sinks can reduce the flow resistance effectively. The optimized geometric structure is obtained by analyzing the effect of geometry parameters on total thermal resistance and pump power, including the slot height, the length of extension area, the distance from the entrance to the extension point and the extension height. The lowest flow resistance is obtained in the composite microchannel and the pump power is 54.9% lower than that of the traditional channel with only 0.7% increase of total thermal resistance. Based on the geometry parameter of the slot height, the distance from the entrance to the extension point and the extension height, two factors are proposed to revise the empirical correlation of friction factor f for the top-slotted and bottom-extension channel. The corrected results are in good agreement with the numerical results.

Thermo-fluidic performance of SiO2–ZnO/water hybrid nanofluid on enhancement of heat transport in a tube: Experimental results

This paper reports an experimental investigation on the convective heat transfer and flow performance of silica-zinc oxide (SiO2–ZnO)/water hybrid nanofluid (HNF) flowing inside a tube with constant heat flux at different Reynolds number (Re) from 7743 to 23,228. A well-stabilized SiO2–ZnO/water HNF with 0.025–0.10weight fractions(φ) was produced via two-step method. Thermal conductivity and viscosity of HNF were measured. The convective heat transfer and flow characteristics of the HNF were determined. Thermal conductivity and viscosity of HNF showed notable enhancements as compared to water. The convective heat transfer coefficient (CHTC) of HNF enhanced with increasing φ and Re. A maximum of 29.44% enhancement in CHTC was obtained with φof 0.10 at Re = 20,131 when compared to water. Further, the friction factor and pressure drop of HNF increased with increasingφ. A maximum of 17.45% increase in friction factor and a maximum increase of 24% in pressure drop were recorded withφ of 0.10 at Re = 23,228 as compared to water. New CHTC and friction factor correlations of high accuracies (R2>90%) have been proposed. A Figure of Merit (FOM) was drawn to describe the thermal performance of HNF. Based on the analysis, the optimum thermal performance was achieved with φ of 0.10 at Re = 20,131.

Air-Side Nusselt Numbers and Friction Factor’s Individual Correlations of Finned Heat Exchangers

Currently, when designing finned heat exchangers (FHE), the average value of the entire heat transfer coefficient (HTC) is considered. However, each row of the heat exchanger (HEX) has different hydraulic-thermal characteristics. The novelty of this research is to present the differentiation of the individual air-side Nusselt number and Darcy-Weisbach friction factor correlations in each row of FHE using CFD modelling. FHE has four-rows, circular tubes, and continuous fins with a staggered tube arrangement. Relationships for the Nusselt number and D-W friction factor derived for the entire exchanger based on CFD modelling were compared with those available in the literature, determined using experimental data. The maximum relative differences between the Nusselt number for a four-row FHE determined experimentally and by CFD modelling are in the range from 22% for a Reynolds number based on a tube outside diameter of 1000 to 30% for a Reynolds number of 13,000. The maximum relative differences between the D-W friction factor for a four-row FHE determined experimentally and by CFD modelling are in the range of 50% for a Reynolds number based on a tube outer diameter of 1000 to 10% for a Reynolds number of 13,000. The CFD modelling performed shows that in the range of Reynolds numbers based on hydraulic diameters from 150 to 1400, the Nusselt number for the first row in a four-row FHE is about 22% to 15% higher than the average Nusselt number for the entire exchanger. In the range of Reynolds number changes based on hydraulic diameter from 2800 to 6000, the Nusselt numbers on the first and second rows of tubes are close to each other. Correlations of Nusselt numbers and D-W friction factors derived for individual tube rows can be used in the design of plate-fin and tube heat exchangers used in equipment such as air-source heat pumps, automotive radiators, air-conditioning systems, and in air hot-liquid coolers. In particular, the correlations can be used to select the optimum number of tube rows in the exchanger.

Friction factor and Reynolds number correlation for finned tube bundle of the air cooler of Monju reactor

Applicability of the analysis model of the cooling ability under a high temperature and low flowrate condition should be improved to evaluate the plant safety in case of the severe accident due to long-term station blackout.The present study reviews experiments on pressure drop behavior for complicated tube bundle geometry to the flow path and then develops a new correlation equation based on a computational fluid dynamics analysis of the Monju air cooler reflecting the actual geometry and plant data.After a basic simulation model being developed for a typical pressure drop experiment, the simulation is applied to the Monju air cooler that has a finned tube bundle. The obtained relationship between friction factors and Reynolds numbers ranging from 10 to 10,000 are fitted to a power function to derive a correlation equation of the fin tube bundle friction factor. The derived correlation equation can estimate pressure loss in the finned tube bundle more precisely than that used in the Monju design. It is applicable to the future reactor design of the air cooler, especially when the cooling ability in low Reynolds number is requested.

Boiling of R290+CF3i mixture inside an offset strip fin plate heat exchanger

In this study, evaporation characteristics of mildly flammable R290 + CF3i mixture (35 % + 65 % by weight) were conducted experimentally in an offset strip fins (OSSF) embedded plate heat exchanger (PHEX). Various operating conditions, namely, saturation temperature (Tsat), heat flux (q), inlet vapor quality (xin), and mass flux (G), were considered in the range of 5–15 °C, 4–10 kW m−2, 0.1–0.8, and 40–80 kg m−2 s−1, respectively. From this study, it was found that nucleate boiling was quenched at lower xin regardless of the operating conditions, although the boiling process was led by convection at high xin. Except at higher G, there was a presence of dry-out at a mean x ranging between 0.36 and 0.50, while the two-phase frictional pressure drop increased steadily with mean x. The heat transfer coefficient was largely augmented with increments in G of 50.15 %, followed by increments in q and T of 15.1 % and 12.3 %, respectively. Similarly, the two-phase frictional pressure drop was increased by 12.3 % and 175.4 % with increases in q and G, respectively, although it diminished by 21.9 % with Tsat. Compared to R1234yf, the mean HTC of R290 + CF3i was 6 % lower with 7 % increased TP-FPD. Additionally, Nusselt number and friction factor correlations have been developed with excellent predictability of 7 % and 12 % of mean absolute error, respectively.