Side Heat Transfer Coefficient Research Articles

CFD Air Flow Evaluation of Finned Tube Evaporator for Refrigerated Display Cabinet Application

This study is aimed to develop a simulation to improve the performance of the finned tube evaporator which is applied to the refrigerated display cabinet. CFD model was developed to be able to analyse the characteristics of air flow inside the fin gap and air side heat transfer coefficient. Geometry of the model of overall finned tube evaporator is considered covering two aluminium wavy fins with an air flow in between, combination of staggered cooper tubes with refrigerant flow inside. Fin gap is designed 4 mm to anticipate frost on the fin surface that can block air flow. Turbulence models used in the study is the realizable k-ε turbulence which had the best performance turbulence model and it was validated with secondary data from previous studies and shows the lowest error only 5.9 %. The use of CFD was found to be sufficiently representative of the heat transfer characteristics of evaporators, and acted as an effective simulation tool to determine the heat transfer coefficient in order to improve efficiency in terms of improved design. The characteristics of air flow between the fin gap and around the tube was obtained various and complex. In the case study the entry velocity of 1.7 m /s at the highest turbulence condition of the first row can reach speeds of 2.75 m/s. Hight turbulence regime in flow can indicate higher the heat transfer coefficient of the evaporator.

Influence of vibration parameters and fin structure parameters on heat transfer performance under vibration conditions

The occurrence of vibration in engineering vehicles during operation is a common phenomenon, and the utilization of vibration-enhanced heat transfer serves as a viable approach to enhance the efficiency of heat transfer in radiators. This study designs a vibration test platform for heat exchangers to investigate the impact of different vibration parameters on heat transfer performance. Additionally, a fin vibration model is constructed using overlapping grid technology to simulate the behavior of flat fins under varying vibration conditions. Corrugated fins were chosen for investigation under vibration conditions, utilizing an orthogonal experimental approach to analyze the impact of fin structural parameters on heat exchanger performance. Findings indicate that vibration enhances the air side heat transfer coefficient of the heat exchanger, in experimental settings, the gas side heat transfer coefficient of the heat exchanger demonstrated a maximum increase of 12.28%. Simultaneously, vibration induces an escalation in pressure loss, with the maximum drop reaching 28.72%. The heat transfer factor j of the heat exchanger exhibits an augmentation in response to heightened vibration intensity, while they diminish with escalating speed. Furthermore, vibration alters the impact of structural factors of corrugated fins on heat transfer efficacy. During vibration, the sensitivity of fin height and wavelength intensifies, whereas the sensitivity of pitch and wave amplitude diminishes. The findings of this research offer valuable insights for further investigation into the heat transfer capabilities of fins under vibration.

Heat transfer in flat tube car radiator with CuO-MgO-TiO2 ternary hybrid nanofluid

An investigation of the performance of a car radiator with CuO-MgO-TiO2 aqueous ternary hybrid nanofluid has been presented to elucidate the effect of nanoparticle concentration, flow rate, and frontal air velocity. The volume concentration of nanoparticles ranges from 0.1% to 0.5%. The effect of fluid inlet temperature, the coolant flow rate and the frontal air velocity on a flat tube radiator were investigated. The effectiveness of the radiator increases with increasing volume concentration as well as coolant flow rate. The coolant side heat transfer coefficient of 0.5% hybrid nano-coolant was 46% higher than the base fluid. The overall heat transfer coefficient was 7.65% higher than the base fluid. The friction factor and the pumping power both decrease with increasing fluid inlet temperature. The pumping power required for nano-coolant is 1.71 times the base fluid. A correlation for the overall heat transfer coefficient is also presented.

Numerical insight into heat transfer enhancement by granules inner-migration in a moving bed: A performance comparison between slanted-stick and plow-shaped agitator

Waste heat recovery of high-temperature granules is one of the most promising sustainable energy supply and carbon reduction ways for industry. A moving bed indirect heat exchanger (MBIHE) with inner-migration was proposed for granular heat recovery. Granule migration and the enhanced heat transfer induced by two types of agitators (i.e., agitator with slanted stick As and with plow-shaped surface Ap) in the MBIHE are analyzed based on DEM coupled with CFD. Owing to the effective agitation, the average heat transfer coefficient in the granule side is enhanced to ∼3 times compared to that without agitation. The heat recovery efficiency in the moving bed reached more than 70% with the agitations of either As or Ap. The heat efficiency of As is ∼7% higher than that of Ap, but with at least 60% greater rotational torque. To ensure reliable agitation, the Ap is suggested to be adopted in the MBIHE to induce granule migration.

Linear and Nonlinear Stability Analysis of Molten Salt Natural Circulation Loop

Abstract A molten salt natural circulation loop (MSNCL) has been setup to study the steady-state, transient, and stability characteristics of molten nitrate salt, mixture of sodium nitrate, and potassium nitrate in 60:40 ratios by weight. Natural circulation experiments in a temperature range of 290 °C–600 °C have been performed. A semi-analytical linear model was derived for stability analysis of rectangular natural circulation loops with conventional localized surface heating and cooling. The developed model includes the effect of wall thermal inertia, variable internal heat transfer coefficient, finite secondary side heat transfer coefficient at cooler, and heat losses for predicting the stability map. These effects are incorporated considering their significant role in modeling high-temperature molten salt-based natural circulation systems. The developed model has been first validated with the experimental data of water loop available in literature. The developed model is then validated with the experimental data generated in MSNCL. Validation of in-house developed nonlinear model has also been performed against the same experimental data. The comparison of both linear and nonlinear stability analysis with the experimental data shows good agreement and articulates the importance of various parameters which have been included in the developed model.

Experimental investigation on heat transfer performance of the grooved spiral wound heat exchanger

To enhance the heat transfer efficiency of the spiral wound heat exchanger, a grooved spiral wound heat exchanger (GSWHE) was proposed. A test platform was accomplished to experimentally investigate the heat transfer performance with the comparison of a spiral wound heat exchanger (SWHE). The uncertainty analysis of the experiment was conducted, with the maximum relative error being not more than ±5%. The comprehensive heat transfer coefficient of the GSWHE is higher than that of the SWHE when the cooling water flow rate is low. To determine the heat transfer coefficient of both sides, the Wilson plot method was applied. The tube side heat transfer coefficient in the SWHE is found to have an error of about ±30% compared to the empirical correlations. The results also clearly suggest that grooves enhance the heat transfer of the shell side in the GSWHE. By employing effectiveness as a criterion, the comprehensive performance of the GSWHE was found to be 1.23 times higher than that of the SWHE.

Flow Characteristics and Heat Transfer Performances of Quadrant Helical Baffle Heat Exchanger with Grooved Joint

Helical baffle heat exchanger has a wide application in many industrial fields because of its high efficiency and low resistance. A new structure of the grooved helical baffle is proposed to strengthen the stability between the overlapped baffles during the spiral flow of fluid. Based on this structure, the liquid water was selected as the working medium in the tube side and shell side, the flow field characteristics and heat transfer performances of the quadrant helical baffle heat exchanger are numerically analyzed using software FLUENT 2017. The results show that when the helical angle is large (50°) and the overlapped amount is small (0.5), the comprehensive performance of the heat exchanger is the best. Therefore, the heat exchanger performance can be improved by choosing the appropriate combination of helical angle and overlapped amount. The correlation formula of the shell side heat transfer coefficient with helical angle, overlapped amount and mass flow rate is obtained by means of regression analysis, which can provide a reference for engineering application and optimal design of grooved helical baffle heat exchanger.

Development of improved correlations for shell side heat transfer co-efficient in multi-tubular reactors network heat exchanger

The multi-tubular reactor takes the form of a shell and tube heat exchanger. The baffles that became of interest were the disk and doughnut baffles as they provided a 15 % greater effectiveness than the commonly used segmental baffle. The shell side heat transfer coefficient is usually predicted by correlations developed by Donohue (1949) and Goyal & Gupta (1984) for disk and doughnut baffles. However, these correlations result in inaccuracies due to the complexity of the configuration. Therefore, Slipcevic (1978), had developed a new method to calculate the shell side heat transfer coefficient. The Slipcevic method takes into consideration the annular flow, the crossflow and the doughnut flow. However, certain portions were gone unconsidered by this method. Therefore, the aim of this study was to modify the Slipcevic method to generate a more accurate shell side heat transfer coefficient. The heat transfer coefficient calculated using the original Slipcevic method was determined to be 58.6 W/m^2. K. The modified method resulted in a coefficient of 49.9 W/m^2. K. The modification resulted in a coefficient 14.8 % less than the original method. Since the simulated value for the propylene process was 15.8 % lower than the Slipcevic procedure, this revealed that the modification was slightly more accurate. The difference of 6.5 % between the two percent differences may be due to inaccuracies resulting from the assumptions of triangular gaps and that the wall viscosities were equivalent to the bulk viscosities.

Numerical study on the thermal-hydraulic characteristic of an automotive parallel-flow condenser under non-uniform airflow

The performance of an automotive parallel-flow condenser was numerically investigated considering both airside and refrigerant-side flow mal-distributions, the thermal-hydraulic characteristic of the condenser under the air-side blockage of each tube pass was first studied, then the influence of the form and degree of uneven airflow that may be revealed in vehicles on the performance of the condenser was analyzed. At last, using the simulation model combined with the measurements of the vehicle in the climatic wind tunnel, the thermal-hydraulic features of the condenser as well as the performance of the A/C system under various front-end status and vehicle conditions was evaluated. It was found that the performance degradation of the condenser resulting from the air-side blockage of the first pass exceeds that caused by the obstruction of all the other three passes together. According to the performance deterioration sequence of the condenser from high to low, the order of the four possible non-uniform airflow patterns in vehicles is low on the top and high at the bottom (LTHB), low in the middle (LM), high in the middle (HM), and high on the top and low at the bottom (HTLB), and the last two forms could even enhance the performance of the condenser under certain working conditions. The rule of cooling airflow optimization is to arrange large/small air velocity on the area with intrinsically large/small refrigerant side heat transfer coefficient of the condenser. The possible direction of cooling airflow design in vehicles is inferred.

Modeling and Optimization of a Micro-Channel Gas Cooler for a Transcritical CO2 Mobile Air-Conditioning System

This study focuses on developing and optimizing of a microchannel gas cooler model for evaluating the performance of a transcritical CO2 mobile air-conditioning system. A simulation model is developed with the aid of MATLAB R2022a. A segment-by-segment modeling approach is utilized by applying the effectiveness-NTU method. State-of-the-art heat transfer and pressure drop correlations are used to obtain air and refrigerant side heat transfer coefficients and friction factors. The developed model is validated through a wide range of available experimental data and is able to predict a gas cooler capacity and pressure drop within an acceptable range of accuracy. The average errors for a gas cooler capacity and pressure drop are 3.79% and 10.24%, respectively. Furthermore, a parametric optimization method is applied to obtain optimal microchannel heat exchanger dimensions, including the number of tubes, microchannel ports, and passes. Different combinations were selected within the practical range to obtain optimal dimensions while keeping the total core volume constant. The simultaneous effect of the number of tubes, the number of ports in each tube, and the number of passes is determined. The objective of the current optimization technique is to minimize the pressure drop for the specific design capacity under different operating conditions without changing the overall volume of the gas cooler. The average pressure drop reduction for the optimal geometry as compared with the baseline geometry under all operating conditions is about 15%. The results from this study can be used to select an optimal geometric design for the required design capacity with a minimal pressure drop without the need for expensive prototype development and testing.

Opportunities for improved space heating energy efficiency from fluid property modifications

Unsteady behaviour of hydronic heating systems causes higher mean room temperatures than are required for comfort. Peak room temperatures depend on interactions between thermostats, heat emitters and the room. The importance of fluid properties on such unsteady heating is often misunderstood meaning potential energy savings are overlooked. This paper demonstrates the influence of fluid modifications and indicates a plausible magnitude of the energy saving opportunity. The results showed that fluid side heat transfer coefficient in isolation had negligible effect. Specific heat capacity of the fluid and flow rates were important, as they altered the amount of embedded energy in the heat emitter when thermostat conditions were met. Reductions in mean heating power for steady demand conditions were between 0 and 7% for plausible changes to fluid properties, depending on heat emitter size, room insulation and external temperature. Reductions in individual cycle energy were between 5 and 25%. When considered in the context of intermittent finite duration heating events, those that contained a small number of thermostat cycles demonstrated energy savings that tended towards the reductions in individual cycle energy. Heating events with larger numbers of cycles showed energy savings tending towards the reduction in mean heating power.

Heat transfer enhancement of moving bed indirect heat exchanger through directional granule migration in 3-D shell enclosure

A huge amount of waste heat is emitted with industrial high-temperature granules. Although the waste heat can be recovered by Moving Bed Indirect Heat Exchanger (MBIHE), the recovery efficiency is limited by extremely large thermal resistance in MBIHE granule side. In order to enhance the heat transfer efficiency, directional granule migration is induced by an optimized agitator to take heat to the Heat Transfer Surface (HTS). The optimized agitator is designed with plow-shaped surface (Ap), which can double the granule migrativity compared to slanted-stick agitator. To further take full advantage of granule migration in 3-directions, a heat exchanger with spherical HTS and embedded Ap is created, and the comprehensive heat transfer coefficient (Kcom) in granule side is significantly enhanced by 383% than conventional MBIHE. The Kcom is immediately increased by ∼75% at agitator start-up and gradually decreased, and the variation of Kcom is closely related to the change rate of migrativity (αc). To achieve cost-effective enhancement for Kcom, the Ap agitation is changed from consecutively to intermittently (50 s agitation with 50 s interval), where the αc is converted from constant to periodic variation, and the energy consumption of Ap can be effectively cut down by 50%.

Mathematical Model of Air Dryer Heat Pump Exchangers

This paper presents a mathematical model of heat pump exchangers and their thermal interaction with a fan for an air dryer. The calculation algorithm developed for the finned heat exchangers is based on the ε-NTU method, allowing the determination of air side and refrigerant side heat transfer coefficients, evaporator and condenser heat capacity and air parameters at the dehumidifier outlet with known exchanger geometries, initial air parameters and mass flow rate. The model was verified on an industrial dehumidifier test bench. This enabled the heat transfer coefficients for the exchanger to be calculated as a function of the speed and, therefore, the power of the fan’s drive motor. An increase in fan performance on the one hand results in an increase in the heat transfer rate, but, on the other hand, it causes an increase in the total energy consumption of the motor. Thus, while it causes an increase in drying capacity, it also causes an increase in the energy consumption of the dehumidifier. In order to optimise the unit in terms of energy consumption, it is therefore necessary to determine a function that relates the amount of heat exchanged to the efficiency of the fan.

Heat transfer and exergy efficiency analysis of 60% water and 40% ethylene glycol mixture diamond nanofluids flow through a shell and helical coil heat exchanger

The thermodynamic, heat transfer and thermal performance factor were analyzed experimentally at different particle volume loadings of water + ethylene glycol mixture based nanodiamond (ND) nanofluids flow in a shell and helical coiled tube heat exchanger. Chemical treatment was used to make ND nanoparticles with an average size of 5.42 nm. The stable ND nanofluids were prepared by considering 60:40% water + ethylene glycol (weight percentage) mixture as base fluid and the experiments were performed at particle loadings ranging from 0.2% to 1.0% and the Reynolds number ranging from 900 to 4800, respectively. In the heat exchanger, the hot fluid passes through the shell side and the cold fluid (nanofluid) passes through the helical coil side. The Wilson plot method is used to analyze the helical coil side heat transfer coefficients. The hot fluid volume flow rate was fixed to 1 lit/min, whereas, the cold fluid (nanofluid) volume flow rate was varied from 2 to 6 lit/min. Results show, at 1.0 vol% and the Reynolds number of 2702, the heat transfer coefficient and Nusselt number is increased by 36.05%, and 27.47%, respectively, over the base fluid. However, the penalty in friction factor and pressure drop is 17.28% and 14.24% under the same Reynolds number and 1.0 vol% nanofluid over the base fluid. Similarly, at 1.0 vol% and Reynolds number of 2702, the thermal entropy generation is dropped by 38.12%, frictional entropy generation is raised by 12.86%, exergy efficiency is enhanced by 36.48% and thermal performance factor is augmented by 1.2094 times over the base fluid.

Investigations on the effect of spacer in direct contact and air gap membrane distillation using computational fluid dynamics

Direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD) have emerged as potential technologies for water and wastewater treatment. This investigation developed a comprehensive two-dimensional computational fluid dynamics model to describe and simulate heat transfer, mass transfer, and fluid flow in DCMD and AGMD processes. The effect of Reynolds number, inlet feed temperature, and distance between two successive spacer filaments on the performance parameters such as mass flux, temperature polarisation, concentration polarisation, and thermal efficiency was investigated in this work. The feed and permeate side heat transfer coefficient increased with Re in the DCMD and AGMD models. The thickness of the velocity boundary layers reduced upon introducing spacers in the feed and permeate channels. The mass flux on the introduction of spacers increased in DCMD and AGMD models because the boundary layer resistance was reduced. The models with spacer showed a lower oncentration polarisation. DCMD model demonstrated a loweroncentration polarisation than the AGMD model as the former has higher mass flux than the latter leading to a higher solute concentration on the membrane surface. The thermal boundary layer was smaller in the spacer-filled channel because of the fluid mixing. In the AGMD model, the permeate side thermal boundary layer was unaffected because of the poor conductivity of air than water.

Thermal energy storage system with a high-temperature nanoparticle enhanced phase change material: Charging and discharging characteristics upon integration with process preheating

Nanoparticle enhanced Phase Change Materials (NePCMs) are modified forms of phase change materials containing nanoparticles such that the thermophysical properties are improved. Solar salt, a phase change material comprising a eutectic mixture of sodium nitrate and potassium nitrate has a thermal conductivity in the range of 0.5–1 W/m K and the addition of nanoparticles to solar salt (nanoparticle enhanced solar salt) has been demonstrated to improve its thermophysical properties. This work evaluates the charging and discharging characteristics of solar salt and nanoparticle enhanced solar salt, with copper nanoparticles (at 0.5 wt%) and reduced Graphene Oxide nanostructures (at 0.5 wt%) individually as additives. Experiments were performed simulating a thermal energy storage system (TES) for process preheating. Silicone oil and Therminol® 55 were used as heat transfer fluids during charging and discharging cycles respectively. The overall heat transfer coefficients of TES containing copper nanoparticle enhanced solar salt and reduced Graphene Oxide nanostructure enhanced solar salt as an energy storage medium were greater than that containing solar salt during both charging and discharging cycles. The enhancements in overall heat transfer coefficients were due to respective enhancements in PCM side heat transfer coefficients caused by thermal conductivity and specific heat increase brought about by the addition of copper nanoparticles/reduced Graphene Oxide nanostructures to solar salt. Considering >10 % enhancements in overall heat transfer coefficient in both charging and discharging cycles with these two NePCMs, latent heat thermal energy storage systems employing them as phase change material will be suitable for integration with a process preheating application.

Experimental comparison and CFD analysis of conventional shell and tube heat exchanger with new design geometry at different baffle intervals

Basically, in the working principle, the transition from high temperature to low temperature is achieved, while the temperature value of the one with the lower temperature rises, this process can continue until the temperature value of the other decreases and reaches equilibrium. Therefore, heat exchangers are highly preferred in the industry where heat transfer is possible, in mass production facilities where nonstop production takes place such as the pharmaceutical and paper industry, and in sectors where energy efficiency is of utmost importance. In the analyses, it is aimed to investigate the changes in the fluid behavior of the conventional one-piece type baffle plate shell and tube heat exchanger at different baffle plate intervals by keeping it constant at different flow rates. Here, water was used as the working fluid to examine the changes in fluid behavior, the direction in which the heat transfer rate per pressure drop changes, the pressure drop and the effects on the body side heat transfer coefficient. As a result, it has been determined that the distance between the baffle plates used in the conventional one-piece type shell and tube heat exchanger varies and the values compared at different flow ranges differ.

Transient analysis of OSU-MASLWR with RELAP5

The present paper deals with the assessment of the original and a modified version of RELAP5/MOD3.3 against the OSU Multi Application Small Light Water Reactor (OSU-MASLWR). The new implemented features regard suitable correlations for the heat transfer coefficient evaluation in helical geometry. Furthermore, two different modelling of the Helical Coil Steam Generator (HCSG) are assessed. In the first approach, HCSG’s primary and secondary sides are collapsed in a single pipe component. In the second model, three equivalent pipes are conceived for the simulation of the three ranks composing the HCSG.Two different power manoeuvring experiments are reproduced. The simulations highlight a satisfactory agreement in both the transients. Nevertheless, the modified code shows enhanced capabilities in the prediction of the HCSG operation. This is due to the improvements adopted in the modified version of RELAP5/MOD3.3 that allows a better modelling of the dryout phenomena occurring within helical tubes, as well as a better estimation of the primary side heat transfer coefficient. The better agreement of the heat exchange is propagated to the primary system, resulting in a more accurate prediction of the inlet and outlet core temperatures, and primary flow rate.

Thermodynamic and hydraulic design characteristics of the fin tube heat exchanger

Heat exchangers are crucial devices that are used to exchange heat between two different fluids: one that is hot and one that is cold. Amongst them, finned tube heat exchangers play a vital role in today’s thermal industry. Due to the increased surface area of the tubes' outer surface (or fins) when utilizing finned tube heat exchangers, the heat transfer rate may be improved as a consequence of the increased surface area of the fins. It is often referred to as a compact heat exchanger in order to highlight the comparatively large heat transfer surface area to volume ratio of the exchanger. They have many vital applications in the field of refrigeration, air conditioning, IC engines, and power plants. The fin and tube heat exchanger has been analyzed for three different cases in the present study: preheater, condenser, and evaporator. The Engineering Equation Solver (EES) software has been used to analyze the heat exchanger for the various thermal, hydrodynamic, and design parameters. Results have been plotted by comparing the various thermal parameters such as overall and inside heat transfer coefficient, Nusselt number, and rate of heat transfer with respect to Reynolds number. Similarly, the hydrodynamic parameters such as Colburn factor, friction factor, and the design parameters such as the number of fins and fin spacing.

Simulation of fin and tube heat exchanger and validation with CFD analysis

Heat exchangers plays significant role in engineering because they transfer heat between two different flow streams and temperatures. There are various designs of heat exchangers based on construction, size, direction of fluid flows. Amongst, industries including oil & gas, power generation, marine, and HVAC&R has the most prevalent application for fin and tube heat exchangers in the broad range. Proper analysis is required prior to the selection of suitable application-specific heat exchanger with good heat transfer characteristics. In the present work, Fin and Tube heat exchanger has been considered for the analysis using ANSYS2021 Fluent. The same model has been simulated and compared with MATLAB Simulink. Variation of heat transfer parameters such as fin side heat transfer coefficients and fin efficiency with mass flow rates and geometric parameters have been observed. Results obtained from the CFD analysis have been validated with mathematical simulation and the reasons for variations in result have been identified and discussed. Some of inferences obtained with both the works has been discussed.