Correlation For Heat Transfer Coefficient Research Articles

Development of a dynamic external CFD and BES coupling framework for application of urban neighbourhoods energy modelling

Current building energy models are weak at representing the interactions between neighbourhoods of buildings in cities. The effect of a neighbourhood on the local microclimate is complex, varying from one building to another, meaning that neighbourhood effects on the airflow around a particular building. A failure to account for this may lead to the miss-calculation of heat transfer and energy demand. Current building energy simulation (BES) tools apply convective heat transfer coefficient (CHTC) correlations, which were developed by using a simplified model of wind flow that neglects neighbourhood effects. Computational Fluid Dynamics (CFD) techniques are able to model these neighbourhood effects and can be used to improve CHTC correlations.This work aims to develop a framework that couples CFD and BES tools to enhance the modelling of outdoor convective heat transfer in different urban neighbourhoods. A dynamic external coupling method was used to combine the benefits from both domains. Firstly, a microclimate CFD model was validated before the coupling stage using wind tunnel data. Secondly, the framework was tested using a benchmark model of a building block. Fully converged values of the surface temperature and CHTC were achieved at each time-step by the BES and CFD domains. The results highlight the importance of neighbourhood effect while the prediction of the hourly averaged external convection using coupling method can amend the simulation by up to 64% comparing to the standalone conventional BES models with DOE-2 CHTC approach.

The influence of microjet array area ratio on heat transfer in the compact heat exchanger

The paper describes the comprehensive study on the effect of microjet array geometrical parameters on the heat transfer enhancement in the modular heat exchanger. The conducted experimental study provides an experimental database on single phase submerged microjet heat transfer. The Wilson plot method was applied to determine the heat transfer coefficients in the laminar and transition flow regimes of a liquid-to-liquid heat exchanger. The heat exchanger was capable of exchanging 296 W of thermal energy at LMTD of 44 K. The obtained heat transfer coefficient reaches over 24,000 W/m2 K.Average Nusselt number predictions of the Wen and Jang (2003) correlation were in best agreement with the experimentally determined average Nusselt numbers. In the whole tested flow range, Nusselt numbers were not well correlated by any of the correlations from the literature. The experimentally determined Nusselt numbers were significantly lower than expected, due to limited applicability of given literature correlations.The author also proposed own experimental correlation for jet impingement heat transfer coefficient, predicting the experimental results within 30%.

Experimental Investigation on Critical Heat Flux for Water Flowing in a Vertical Uniformly Heated Rifled Tube under Near-critical Pressures

This study experimentally investigated the critical heat flux (CHF) of departure from nucleate boiling (DNB) of water flowing under near-critical pressures in a 2 m-long vertical upward rifled tube with the size of ϕ35 × 5.67 mm. Operating conditions included pressures of 18–21 MPa, mass fluxes of 475–1000 kg·m−2·s−1, inlet subcooling temperatures of 3–5°C, and wall heat fluxes of 40–960 kW·m−2. Tube wall temperature distribution and heat transfer performance in different test conditions were obtained. The effects of the operating parameters on CHF were analyzed. Four heat transfer coefficient correlations were evaluated against our experimental data for further investigation of the two-phase heat transfer characteristics. A heat transfer correlation based on Martinelli number utilized in two-phase region and two empirical correlations used to predict the CHF in rifled tube at near-critical pressures were proposed. Meanwhile, experimental CHF data in rifled tube were compared with six widely used correlations and a CHF look-up table. The CHF enhancement effect in rifled tube is obvious as compared with the CHF data in smooth tube. Results show that DNB occurs at low vapor quality and subcooled region in the rifled tube at near-critical pressures. The increase in pressure leads to the early occurrence of DNB and the decrease in CHF, whereas the increase in mass flux delays the occurrence of DNB and results in the increase in CHF. DNB presents a tendency to move toward the inlet of the rifled tube. At individual operating conditions, DNB and dryout coexist at different sections of the rifled tube.

An experimental investigation of flow boiling heat transfer coefficient and pressure drop of R410A in various minichannel multiport tubes

The present work demonstrates the two-phase flow boiling heat transfer coefficient and pressure drop of R410A inside various minichannel multiport tubes. The experimental investigation was performed in four tube types with the hydraulic diameters of 1.16, 1.14, 1.07 and 0.96 mm and the number of parallel channels are 7, 11, 16 and 9, respectively. The data were conducted with the heat fluxes of 3 and 6 kW m−2, the mass flux ranging from 50 to 500 kg m−2 s−1, the saturation temperature of 6 °C and the vapor quality up to 1.0. The heat transfer coefficient of R410A was found to be affected by the heat flux, mass flux, vapor quality and the geometry of channel. The frictional pressure drop gradient was also affected by the mass flux and vapor quality but independent with the variation of heat flux. In addition, the present data were compared with various heat transfer coefficient correlations and pressure drop models in literature. Finally, new correlation of heat transfer coefficient was proposed based on the present experimental data.

Condensation heat transfer characteristics of R245fa in a shell and plate heat exchanger for high-temperature heat pumps

In this study, the condensation heat transfer characteristics of R245fa in a shell and plate heat exchanger (SPHE) are measured and analyzed by varying the mean vapor quality from 0.16 to 0.86, mass flux from 16.0 to 45.0 kg m−2 s−1, heat flux from 1.3 to 9.0 kW m−2, and saturation temperature from 90 to 120 °C for high-temperature heat pumps (HTHPs). The condensation heat transfer coefficient and frictional pressure drop are shown to increase with increasing mean vapor quality and mass flux and with decreasing saturation temperature. The condensation heat transfer coefficient of R245fa in SPHE is on average 5.9% lower than that in the brazed plate heat exchanger (BPHE) with a similar effective heat transfer area. However, the average two-phase frictional pressure drop of R245fa in SPHE is 16.7% lower than that in BPHE. Empirical correlations for the condensation heat transfer coefficient and two-phase frictional pressure drop in SPHE are developed with mean absolute errors of 8.2% and 9.4%, respectively.

Flow condensing heat transfer of R410A, R22, and R32 inside a micro-fin tube

ABSTRACT An experimental study of condensation heat transfer characteristics of flow inside horizontal micro-fin tubes is carried out using R410A, R22, and R32 as the test fluids. This study especially focuses on the influence of heat transfer area upon the condensation heat transfer coefficients. The test sections were made of double tubes using the counter-flow type; the refrigerants condensation inside the test tube enabled heat to exchange with cooling water that flows from the annular side. The saturation temperature and pressure of the refrigerants were measured at the inlet and outlet of the test sections to defined state of refrigerants, and the surface temperatures of the tube were measured. A differential pressure transducer directly measured the pressure drops in the test section. The heat transfer coefficients and pressure drops were calculated using the experimental data. The condensation heat transfer coefficient was measured at the saturation temperature of 48°C with mass fluxes of 50–380 kg/(m2s) and heat fluxes of 3–12 kW/m2. The values of experimental heat transfer coefficient results are compared with the predicted values from the existing correlations in the literature, and a new condensation heat transfer coefficient correlation is proposed.

Correlation of heat transfer coefficient for two-component two-phase slug flow in a vertical pipe

The prediction of heat transfer coefficient for gas–liquid two-phase flows in vertical pipes is important for designing many industrial and engineering apparatuses such as nuclear power plants, solar collectors and petroleum pipelines. The mechanism of two-component two-phase heat transfer is more complicated than that of single-phase heat transfer. This study aims at developing a robust correlation of heat transfer coefficient for two-component two-phase slug flows in vertical pipes. The theoretical framework for the correlation development is inspired by considering classic Reynolds and Chilton–Colburn analogies. The framework demonstrates that the functional dependence of the two-component two-phase heat transfer coefficient on a void fraction or a two-phase multiplier is expressed by the Reynolds and Chilton–Colburn analogies. The correlation for the heat transfer coefficient of two-component two-phase slug flows in vertical pipes has been developed and validated by more than 200 existing data. The newly developed correlation predicts 95.1% of the collected two-phase heat transfer data within ± 30% error and the mean absolute relative deviation of the correlation is estimated to be 14.2%. The extended applicability of the newly developed correlation to other flow regimes such as bubbly, churn and annular flow regimes has been verified by the existing experimental data. The possible application of the newly developed correlation to large pipes is also discussed. In summary, it is expected that the newly developed correlation can predict the heat transfer coefficient of two-component two-phase flows under various conditions such as vertical upward and downward flows, developing and fully developed flows, laminar and turbulent flows and all two-phase flow regimes. The application of the correlation to vertical large pipes is also considered to be reasonable.

Experimental and Analytical Investigation of Heat Transfer Coefficient of a Water Cooled Condenser for Different Water Flows and Condensation Pressures

In the phase change process, latent heat is transferred and the amount of heat transferred will be excessive high compared to the amount of sensible heat transfer. For that reason, condensation and evaporation processes are the main steps in the refrigeration cycle in order to increase the amount of heat transferred. The main objective of this study is to experimentally and analytically examine overall heat transfer coefficient and to present the effect of water flow and refrigerant pressure on condensation process. For this purpose, a refrigeration system where a water cooled condenser with a heat transfer surface area of 0.075 m2 was installed. R134a was used as a refrigerant and condenses on the outer surface of the pipe that water circulates through. In this study, experiments were repeated for water mass flow rates of 15, 20, 25, 30 and 35 g/s at constant 7.0 bar condensation pressure. Then, condensation pressures were changed to 6.5, 6.75, 7.0, 7.25 and 7.5 bar at constant water flow rates of 25 g/s. Correlation of condensation heat transfer coefficient has been applied to refrigerant side of the condenser unit. On the other side, logarithmic mean temperature difference (LMTD) and number of transfer unit (ε-NTU) methods have been applied to experimental results in order to calculate heat transfer coefficient. Experimental results for different water flow rates at constant refrigerant pressure and for different condensation pressures at constant water flow rate are taken from the refrigeration machine unit and have been compared with calculated values that are obtained from condensation heat transfer correlation.

Experimental investigation of circumferentially non-uniform heat flux on the heat transfer coefficient in a smooth horizontal tube with buoyancy driven secondary flow

In this experimental investigation the influence of non-uniform heat flux distributions on the internal heat transfer coefficient in a horizontal circular tube was studied for liquid water. The tube had an inner diameter of 27.8 mm and a length to diameter ratio of 72. Different outer wall heat flux conditions were studied for Reynolds numbers ranging from 650 to 2600 at a Prandtl number of approximately 6.5. Heat flux distributions included fully uniform heating (which had a circumferential angle span of 360°) and different partial uniform heat flux distributions with angle spans of 180° or 90° at different circumferential positions. Depending on the angle span, local heat flux intensities ranging from 1658 W/m2 to 6631 W/m2 were tested. Results indicate that the average steady state Nusselt number is greatly influenced by the applied heat flux position and intensity. Highest average heat transfer coefficients were achieved for cases where the applied heat flux was positioned on the lower half (in terms of gravity) of the tube circumference, while the lowest heat transfer coefficients were achieved when the heating was applied to the upper half of the tube. Smaller angle spans produced lower heat transfer coefficients. The relative thermal performance of the different heating scenarios where characterised and described by means of newly developed heat transfer coefficient correlations for angle spans of 180° and 90° which correlated 92% and 96% of the data respectively within 3% of the measured Nusselt number.

Development and validation of a CO2 gas cooler moving-boundary model.

This paper presents the development of a CO2 gas cooler model using the moving-boundary method. The model aims to separate the gas cooler into two regions: supercritical gas and supercritical liquid by means of the CoolProp thermophysical property library (Bell et al., 2014). The model uses the latest correlations for refrigerant and air-side heat transfer coefficients and pressure drops. The experimental results from Ge and Cropper (2009) are used for model validation. A total of at 36 operating conditions have been reported. The model predicted the gas cooler heating capacity within an MAE of ±4.7% and refrigerant side outlet temperature within ±3 K. The present moving-boundary model also showed an improved agreement compared to the segment-by-segment model proposed by Ge and Cropper (2009).

Heat transfer correlation for two-component two-phase slug flow in horizontal pipes

Two-component gas-liquid two-phase slug flow in horizontal pipes occurs frequently in many industrial applications. The flow and heat transfer characteristics are complicated due to the intermittent flow structures. In view of the importance of predicting the heat transfer characteristics of the slug flow, this present study aims at developing a semi-theoretical heat transfer correlation for two-component two-phase slug flow based on the concept of Reynolds and Chilton-Colburn analogies. Firstly, an extensive literature review on existing databases and correlations of heat transfer coefficient for two-component two-phase slug flow was conducted. More than 500 experimental data and 8 heat transfer correlations were collected. The comparison between collected database and correlations indicated that none of the correlations could estimate the whole database satisfactorily. The relationship between heat transfer and pressure drop was investigated theoretically and an improved semi-theoretical heat transfer correlation was developed based on Reynolds and Chilton-Colburn analogies and the collected experimental results from 16 sources. The comparison analysis demonstrated that the newly-developed correlation achieved an excellent predictive capability with a wide range of test conditions. The newly developed correlation demonstrated that 91.5% of the data was predicted within ±30% error with the mean absolute relative deviation of 14.0%. In addition, the extension of the new correlation to other horizontal two-phase flow regimes was discussed. The newly-developed semi-theoretical correlation would be useful for predicting heat transfer coefficient of two-component two-phase flow in horizontal pipes.

Direct numerical simulation of heat transfer from a cylinder immersed in the production and decay regions of grid-element turbulence

The present direct numerical simulation (DNS) study, the first of its kind, explores the effect that the location of a cylinder, immersed in the turbulent wake of a grid-element, has on heat transfer. An insulated single square grid-element is used to generate the turbulent wake upstream of the heated circular cylinder. Due to fine-scale resolution requirements, the simulations are carried out for a low Reynolds number. Three locations downstream of the grid-element, inside the production, peak and decay regions, respectively, are considered. The turbulent flow in the production and peak regions is highly intermittent, non-Gaussian and inhomogeneous, while it is Gaussian, homogeneous and fully turbulent in the decay region. The turbulence intensities at the location of the cylinder in the production and decay regions are almost equal at 11 %, while the peak location has the highest turbulence intensity of 15 %. A baseline simulation of heat transfer from the cylinder without oncoming turbulence was also performed. Although the oncoming turbulent intensities are similar, the production region increases the stagnation point heat transfer by 63 %, while in the decay region it is enhanced by only 28 %. This difference cannot be explained only by the increased approaching velocity in the production region. The existing correlations for the stagnation point heat transfer coefficient are found invalid for the production and peak locations, while they are satisfied in the decay region. It is established that the flow in the production and peak regions is dominated by shedding events, in which the predominant vorticity component is in the azimuthal direction. This leads to increased heat transfer from the cylinder, even before vorticity is stretched by the accelerating boundary layer. The frequency of oncoming turbulence in production and peak cases also lies close to the range of frequencies that can penetrate the boundary layer developing on the cylinder, and therefore the latter is very responsive to the impinging disturbances. The highest Nusselt number along the circumference of the cylinder is shifted 45 degrees from the front stagnation point. This shift is due to the turbulence-generating grid-element bars that result in the prevalence of intense events at the point of maximum Nusselt number compared to the stagnation point.

Flow Boiling Heat Transfer of R134a in a Vertical Helically Coiled Tube

ABSTRACTThe boiling heat transfer characteristics of R134a in a helically coiled tube under high heat flux condition were investigated experimentally in present paper. The inner diameter and the coil diameter of the coil were 8 mm and 205 mm, respectively. Experiments were carried out with the heat flux in the range of 10–60 kW/m2, pressure 0.8–1.1 MPa, mass flux 195–400 kg/(m2 s) and vapor qualities 0.1–0.9. The results show that nucleate boiling plays a significant role even at high vapor quality. Inversely, the convective boiling was weakened in the high quality region under high heat flux conditions. The available boiling heat transfer coefficient correlations were compared with present experimental results. A new boiling heat transfer coefficient correlation for R134a in helically coiled tube was proposed based on the boiling mechanisms analysis. The new correlation was also proved to be applicable for high mass flux and low heat flux conditions.

Numerical study on shell-side saturated boiling heat transfer in spiral wound heat exchanger

A model was established to explore the shell-side heat transfer characteristic of saturated boiling in spiral wound heat exchanger (SWHE), which was used for liquefied natural gas (LNG). The saturated boiling mechanism was investigated by ANSYS CFX based on the model which was validated by the experimental data, and the correlations for shell-side heat transfer coefficient of SWHE were improved, where mass flux G, heat flux q and vapor quality x were 40–100 kg/(m2·s), 2,000–8,000 W/m2 and 0.3–0.9, respectively. The results show that the deviations between simulation results and experimental data are within ±13%. According to the simulation results, the main boiling mechanism for shell-side saturated boiling heat transfer is forced convective, and the contribution of nucleate boiling contribution is slight (less than 0.77%). At the same time, the improved correlations are in good accuracy, the deviations between correlation calculation results and simulation results are within ±25%.

Experimental comparative evaluation of a graphene nanofluid coolant in miniature plate heat exchanger

As a novel coolant, the ethylene glycol-water (50 wt.%:50 wt.%) with graphene nanoplatelets nanofluids (GnP-EGW) were prepared at four weight concentrations (0.01, 0.1 0.5 and 1.0 wt.%), and heat transfer and pressure drop characteristics in a miniature plate heat exchanger (MPHE) were investigated. All nanofluid samples were prepared and diluted by ultrasonic vibration, and their thermal conductivity and dynamic viscosity were measured by a transient plane source method and a rotational rheometer, respectively. Firstly, the convective heat transfer coefficient (HTC) and pressure drop correlations were predicted under the condition that water was employed as working fluid in both the hot and cold sides of the MPHE. Then, the effects of GnP concentrations of nanofluids on the thermal and hydraulic performances have been determined for the MPHE with the nanofluid in hot side and the water in cold side. Parametric evaluation and performance comparison of the MPHE using GnP-EGW were analyzed via various operating conditions. Experimental analysis showed that: the proposed correlations from water can predict the experimental data of the base fluid and GnP-EGW nanofluids. In the proper concentration range from 0.01 to 0.1 wt.%, the GnP-EGW nanofluid has an acceptable pressure drop penalty but a higher heat transfer performance compared with the base fluid in the MPHE, which reveals that it might be a potential cooling medium.

Development and validation of a semi-empirical model for two-phase heat transfer from arrays of impinging jets

Two-phase jet impingement is a compact cooling technology that provides high-heat-flux dissipation at manageable pressure drop, with applications in cooling power electronics and server modules. The extensive set of geometrical parameters and operating conditions that determine the heat transfer behavior of jet impingement systems provide an attractive level of design flexibility. In the present study, a semi-empirical approach is developed to predict heat transfer from arrays of jets of liquid that undergoes phase change upon impingement. In the modeling approach developed, the jet array is divided into unit cells centered on each orifice that are assumed to behave identically. Based on prior experimental observations, the impingement surface in each unit cell is divided into two distinct regions: a single-phase heat transfer region directly under the jet, and a surrounding boiling heat transfer region along the periphery. Single-phase convection and boiling heat transfer correlations available in the literature are used to estimate the heat transfer coefficient distribution in each region, and the mean surface temperature of the unit cell is estimated via area-averaging. An analysis is performed to show that the model outputs are sensitive to the heat transfer coefficient correlations used as inputs, with the choice depending on the heat flux input and the expected operating regime. Experiments are performed to validate the area-averaged thermal performance predictions. The model results are also compared against experimental data in the literature. The semi-empirical modeling approach developed in this work successfully represents the different heat transfer modes and transitions that occur during two-phase jet impingement. The location of transition to boiling predicted by the model is consistent with prior experimental observations of an inward-creeping boiling front with increasing heat flux. The predicted temperature difference between the surface and the jet inlet across all experimental comparisons has a mean absolute percentage error of 3.88%. The proposed modeling approach is demonstrated to be a practical tool in the development of two-phase jet array impingement devices, allowing for parametric exploration across the expansive design space.

Numerical modeling of flow film boiling in cryogenic chilldown process using the AIAD framework

Flow film boiling plays a dominant role in cryogenic chilldown process, which involves complicated heat transfer and flow regime transition. Nevertheless, existing researches about flow film boiling with cryogenic fluids are relatively limited. In this study, a Computational Fluid Dynamics (CFD) model based on a wall heat flux partition algorithm is built. The AIAD framework implemented in the two-fluid model is employed to appropriately calculate the drag force on the liquid-vapor interfaces. The CFD model is validated by the satisfactory coincidence between the simulated heat fluxes and experimental data in literature. Accordingly, the two-phase interaction on the flow regime and heat transfer is further investigated. The results reveal that the vapor film beneath the bulk liquid becomes thinner due to the drag force on the liquid-vapor interface. In addition, FFT analysis on the pressure drop shows that dominant frequency of the interfacial waves in the tube mainly locates around 2.8 Hz. The normalized intensity indicates that fluctuation becomes more violent with the increase of superheat and inlet liquid flow rate. Finally, comparison between correlations and experimental data indicates that a correlation of heat transfer coefficient considering both film boiling effect and forced convective flow effect needs to be proposed.

비응축성 기체 존재 시 수직관 외벽에서의 응축 열전달계수 상관식 개발

비응축성 기체 존재 시 수직관 외벽에서의 응축 열전달계수 상관식 개발

Investigation of boiling heat transfer characteristics of R134a flowing in smooth and microfin tubes

In this study, boiling heat transfer and pressure drop experiments are performed in order to investigate heat transfer enhancement of microfin tube comparing with smooth one. During constant heat flux (10 kW m−2) experiments, effect of mass flux (190–381 kg m−2 s−1), saturation temperature (15–22 °C) and vapor quality (0.21–0.77) on heat transfer coefficient and pressure drop are investigated. In addition, boiling heat transfer coefficient and frictional pressure drop correlations given in the literature are compared with measured ones. Moreover, the experiments revealed that experimental boiling heat transfer coefficient and total pressure drop of R134a flowing microfin tube are 1.9 and 3.0 times higher than smooth one having the same fin root diameter, respectively. Furthermore, 94 number of heat transfer and pressure drop raw experimental data belonging to the microfin tube are given for researchers to validate their theoretical models.

Enhancing the thermosiphon-driven discharge of a latent heat thermal storage system used in a personal cooling device

Personal cooling devices reduce energy loads by allowing buildings to operate with elevated setpoint temperatures, without compromising on the occupant comfort. One such novel technology called the Roving Comforter (RoCo) uses a compact R134a based vapor compression system for cooling. Following its cooling operation, during which waste heat from the condensing refrigerant is stored in a phase change material (PCM), a two-phase loop thermosiphon is used to discharge (solidify) the PCM to enable its next operation. The transient operation of this thermosiphon is the focus of the present article. Use of a PCM as the storage medium provides high energy density due to the ability to store thermal energy as latent heat during the phase transition; however, the discharge rate is limited by the low thermal conductivity of the PCM. Insertion of a graphite foam within the PCM can increase the rate of discharge and decrease the downtime of the cooling device. Since graphite enhancement involves a tradeoff between improving the discharge time at the expense of PCM volumetric latent heat, the impact of graphite foam density on the PCM discharge rate is investigated by using a Modelica-based transient model of the thermosiphon. The semi-empirical model, which uses relevant heat transfer coefficient and pressure drop correlations for both refrigerant and airside heat transfer, captures the complex phenomena involving simultaneous phase change of the refrigerant and the PCM. The graphite enhanced PCM selected from this analysis results in a 51% reduction in the discharge time with addition of only 5% to the thermal storage weight, without compromising the required cooling time.