Numerical simulation of heat transfer to separation air flow in an annular pipe

Separation and reattachment of air flow through a sudden expansion in an annular passage are considered in this study. Backward facing steps play a vital role in the design of many heat related applications where heat transfer is concerned. In the present work, numerical simulation is performed using computer fluid dynamics package (Fluent) to study the effect of step flow in an annular passage. The results are compared with the preliminary experimental findings. In the study, the flowing fluid was considered heated uniformly from the beginning of the expansion. Constant heat flux approach was also considered for the heat transfer investigation. Annular pipe flow system having a step ratio of D/d = 1.8 was considered where d and D are representing the diameter of the pipe before and after expansion. Numerical simulation review shows that the reattachment point extends further with the increase of velocity for different occasions. Finally, the local Nusselt number (Nu) in separation flow increases with the increase of Reynolds number (Re).     

Experimental analysis for the determination of the convective heat transfer coefficient by measuring pressure drop directly during annular condensation flow of R134a in a vertical smooth tube

This study investigated the direct relationship between the measured condensation pressure drop and convective heat transfer coefficient of R134a flowing downward inside a vertical smooth copper tube having an inner diameter of 8.1 mm and a length of 500 mm during annular flow. R134a and water were used as working fluids on the tube side and annular side of a double tube heat exchanger, respectively. Condensation experiments were performed at mass fluxes of 260, 300, 340, 400, 456 and 515 kg m⁻² s⁻¹ in the high mass flux region of R134a. The condensing temperatures were around 40 and 50 °C; the heat fluxes were between 10.16 and 66.61 kW m⁻². Paliwoda’s analysis, which focused mainly on the determination of the two-phase flow factor and two-phase length of evaporators and condensers, was adapted to the in-tube condensation phenomena in the test section to determine the condensation heat transfer coefficient, heat flux, two-phase length and pressure drop experimentally by means of a large number of data points obtained under various experimental conditions.     

The effects of different entrance sections lengths and heating on free and forced convective heat transfer inside a horizontal circular tube

An experimental study was done for hydrodynamically fully developed and thermally developing laminar air flows in a horizontal circular tube has a 30 mm inside diameter and 900 mm heated length (L/D = 30) under a constant wall heat flux boundary condition, with different aluminum entrance section pipes (calming sections) having the same inside diameter as test section pipe but with variable lengths of 600 mm (L/D = 20), 1200 mm (L/D = 40), 1800 mm (L/D = 60), and 2400 mm (L/D = 80). The Reynolds number ranged from 400 to 1600 and the heat flux is varied from 60 W m^− 2 to 400 W m^− 2. This paper examines the effects of the entrance sections lengths and heating on the free and forced convection heat transfer process. The surface temperature data were measured and heat transfer rates at different heat flux levels as well as different Reynolds numbers were calculated and correlated in the form of relevant parameters. The buoyancy force has a significant effect on the heat transfer and the combined convection factor was approximately varied form 0.13 ≤ Gr/Re² ≤ 7.125. It was found that the surface temperature increases as the entrance section length increases. It was inferred that the heat transfer decreases as the entrance section length increases due to the flow resistance and the mass flow rate. The proposed correlation was compared with available literature and with laminar forced convection and showed satisfactory agreement.     

Experimental investigation of heat transfer coefficient of R134a during condensation in vertical downward flow at high mass flux in a smooth tube

The two-phase heat transfer coefficients of pure HFC-134a condensing inside a smooth tube-in-tube heat exchanger are experimentally investigated. The test section is a 0.5 m long double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is constructed from smooth copper tubing of 9.52 mm outer diameter and 8.1 mm inner diameter. The test runs are performed at average saturation condensing temperatures between 40–50 °C. The mass fluxes are between 260 and 515 kg m^− 2s^− 1 and the heat fluxes are between 11.3 and 55.3 kW m^− 2. The quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The average heat transfer coefficient of the refrigerant is determined by applying an energy balance based on the energy transferred from the test section. The effects of heat flux, mass flux and condensation temperature on the heat transfer coefficients are also discussed. Eleven well-known correlations for annular flow are compared to each other using a large amount of data obtained from various experimental conditions. A new correlation for the condensation heat transfer coefficient is proposed for practical applications.     

Two-phase flow in high-heat-flux micro-channel heat sink for refrigeration cooling applications: Part II—heat transfer characteristics

This paper is the second of a two-part study concerning two-phase flow and heat transfer characteristics of R134a in a micro-channel heat sink incorporated as an evaporator in a refrigeration cycle. Boiling heat transfer coefficients were measured by controlling heat flux (q″ = 15.9 − 93.8 W/cm²) and vapor quality (x_e = 0.26 − 0.87) over a broad range of mass velocity. While prior studies point to either nucleate boiling or annular film evaporation (convective flow boiling) as dominant heat transfer mechanisms in small channels, the present study shows heat transfer is associated with different mechanisms for low, medium and high qualities. Nucleate boiling occurs only at low qualities (x_e < 0.05) corresponding to very low heat fluxes, and high fluxes produce medium quality (0.05 < x_e < 0.55) or high quality (x_e > 0.55) flows dominated by annular film evaporation. Because of the large differences in heat transfer mechanism between the three quality regions, better predictions are possible by dividing the quality range into smaller ranges corresponding to these flow transitions. A new heat transfer coefficient correlation is recommended which shows excellent predictions for both R134a and water.     

Prediction of dryout and post-dryout heat transfer at high pressures using a one-dimensional three-fluid model

A set of closure relations, in the context of a one-dimensional three-fluid model, is presented for the prediction of dryout and post-dryout heat transfer at high pressure (P/P_cr>0.3) conditions. It is shown that the traditional models based on low pressure data for interfacial friction, droplet size and the transition criteria for onset of annular flow cannot be readily extended to high pressure situations. The proposed new relations are validated by comparing with literature data in the pressure range of 30-200 bar, mass flux range of 500-3000 kg m⁻² s⁻¹ and tube inner diameters in the range of 10-25 mm. Good agreement is obtained for the dry out quality and the tube wall temperature in the post-dry out region except for cases of low mass flux at high pressures. The predictions show that at high pressure, high mass flux conditions, annular flow may prevail for low gas phase volume fraction, the droplet and the liquid film volume fractions being an order of magnitude higher than those encountered in typical air-water experiments.     

Artificial neural network techniques for the determination of condensation heat transfer characteristics during downward annular flow of R134a inside a vertical smooth tube

In this study, the best artificial intelligence method is investigated to estimate the measured convective heat transfer coefficient and pressure drop of R134a flowing downward inside a vertical smooth copper tube having an inner diameter of 8.1 mm and a length of 500 mm during annular flow numerically. R134a and water are used as working fluids in the tube side and annular side of a double tube heat exchanger, respectively. The ANN training sets have the experimental data of in-tube condensation tests including six different mass fluxes of R134a such as 260, 300, 340, 400, 456 and 515 kg m^− 2 s^− 1, two different saturation temperatures of R134a such as 40 and 50 °C and heat fluxes ranging from 10.16 to 66.61 kW m^− 2. The quality of the refrigerant in the test section is calculated considering the temperature and pressure obtained from the experiment. The pressure drop across the test section is directly measured by a differential pressure transducer. Input of the ANNs are the measured values of test section such as mass flux, heat flux, the temperature difference between the tube wall and saturation temperature, average vapor quality, while the outputs of the ANNs are the experimental condensation heat transfer coefficient and measured pressure drop in the analysis. Condensation heat transfer characteristics of R134a are modeled to decide the best approach using several artificial neural network (ANN) methods such as multilayer perceptron (MLP), radial basis networks (RBFN), generalized regression neural network (GRNN) and adaptive neuro-fuzzy inference system (ANFIS). Elimination process of the ANN methods is performed by means of 183 data points, divided into two sets randomly, obtained in the experiments. Sets of test and training/validation include 33 and 120/30 data points respectively for the elimination process. Validation process, in terms of various experimental conditions, is done by means of 368 experimental data points having 68 data points for test set and 300 data points for training set. In training phase, 5-fold cross validation is used to determine the best value of ANNs control parameters. The ANNs performances were measured by means of relative error criteria with the usage of unknown test sets. The performance of the method of multi layer perceptron (MLP) with 5-13-1 architecture and radial basis function networks (RBFN) were found to be in good agreement, predicting the experimental condensation heat transfer coefficient and pressure drop with their deviations being within the range of ± 5% for all tested conditions. Dependency of outputs of the ANNs from input values is also investigated in the paper.     

Convective boiling heat transfer characteristics of CO2 in microchannels

Convective boiling heat transfer coefficients and dryout phenomena of CO₂ are investigated in rectangular microchannels whose hydraulic diameters range from 1.08 to 1.54 mm. The tests are conducted by varying the mass flux of CO₂ from 200 to 400 kg/m² s, heat flux from 10 to 20 kW/m², while maintaining saturation temperature at 0, 5 and 10 °C. Test results show that the average heat transfer coefficient of CO₂ is 53% higher than that of R134a. The effects of heat flux on the heat transfer coefficient are much significant than those of mass flux. As the mass flux increases, dryout becomes more pronounced. As the hydraulic diameter decreases from 1.54 to 1.27 mm and from 1.27 to 1.08 mm at a heat flux of 15 kW/m² and a mass flux of 300 kg/m² s, the heat transfer coefficients increase by 5% and 31%, respectively. Based on the comparison of the data from the existing models with the present data, the Cooper model and the Gorenflo model yield relatively good predictions of the measured data with mean deviations between predicted and measured data of 21.7% and 21.2%, respectively.     

Forced convection heat transfer from a circular cylinder in crossflow to air and liquids

Local and average heat transfer by forced convection from a circular cylinder is studied for Reynolds number from 2 × 10³ to 9 × 10⁴ and Prandtl number from 0.7 to 176. For subcritical flow, the local heat transfer measurement indicates three regions of flow around the cylinder: laminar boundary layer region, reattachment of shear layer region and periodic vortex flow region. The average heat transfer in each region is calculated and correlated with the Reynolds number and the Prandtl number. The Nusselt number in each region strongly depends on the Reynolds number and the Prandtl number with different power indices. An empirical correlation for predicting the overall heat transfer from the cylinder is developed from the contributions of heat transfer in these three regions.     

New proposed two-phase multiplier and evaporation heat transfer coefficient correlations for R134a flowing at low mass flux in a multiport minichannel

New correlations of the two-phase multiplier and heat transfer coefficient of R134a during evaporation in a multiport minichannel at low mass flux are proposed. The experimental results were obtained from a test using a counter-flow tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water in the gap between the outer and inner tubes. Test section is composed of the extruded multiport aluminium inner tube with an internal hydraulic diameter of 1.2 mm and an acrylic outer tube with an internal hydraulic diameter of 25.4 mm. The experiments were performed at heat fluxes between 10 and 35 kW/m², and a refrigerant mass flux between 45 and 155 kg/(m² s). Some physical parameters that influenced the frictional pressure drop and heat transfer coefficient are examined and discussed in detail. The pressure drop and heat transfer coefficient results are also compared with existing correlations. Finally, new correlations for predicting the frictional pressure drop and heat transfer coefficient at low mass fluxes are proposed.     

Local turbulent opposing mixed convection heat transfer in inclined flat channel for stably stratified airflow

Local turbulent mixed convection heat transfer in inclined flat channels (?=20-90° from horizontal position) for opposing flows was investigated for the case when only upper wall is heated (under stably stratified flow conditions). Wide ranges of airflow parameters are covered: Re=4 × 10³-4 × 10⁴, Gr_q=1.7 × 10⁸-1.4 × 10¹⁰, pressures; p=0.2; 0.4; 0.6; 0.8 MPa. Based on analysis of local heat transfer data and existing information in the literature three characteristic regions in the buoyancy parameter range investigated were identified: region without buoyancy instabilities, transition region and region with buoyancy instabilities in whole heated section. For the region without buoyancy instabilities correlation for calculation of heat transfer in inclined flat channels was suggested.     

Boiling heat transfer characteristics of R-113 in a vertical small diameter tube under natural circulation condition

Boiling heat transfer characteristics of freon R-113 are experimentally investigated in a vertical small diameter tube, D=1.45 mm and L=100 mm at a wide pressure range of 19-269 kPa under natural circulation condition. Except the entrance region of the test section, the flow regime is annular in view of the measured vapor flux. The pool boiling correlations of Stephan and Abdelsalam and McNelly equally well predict the experimental data within an error of ±20%. No enhancement of heat transfer coefficient is obtained although D/B is less than 1.5, which differs from the finding of Klimenko.     

Experimental study of forced and free convective heat transfer in the thermal entry region of horizontal concentric annuli

Forced and free convective heat transfer for thermally developing and thermally fully developed laminar air flow inside horizontal concentric annuli in the thermal entrance length has been experimentally investigated. The experimental setup consists of a stainless steel annulus having a radius ratio of 2 and an inner tube with a heated length of 900 mm subjected to a constant wall heat flux boundary condition and an adiabatic outer annulus. The investigation covers Reynolds number range from 200 to 1000, the Grashof number was ranged from 6.2 × 10⁵ to 1.2 × 10⁷. The entrance sections used were long tube with length of 2520 mm (L/D_h = 63) and short tube with length of 504 mm (L/D_h = 12.6). The surface temperature distribution along the inner tube surface, and the local Nusselt number distribution versus dimensionless axial distance Z_t were presented and discussed. It is inferred that the free convection effects tended to decrease the heat transfer at low Re number while to increase the heat transfer for high Re number. This investigation reveals that the Nusselt number values were considerably greater than the corresponding values for fully developed combined convection over a significant portion of the annulus. The average heat transfer results were correlated in terms of the relevant dimensionless variables with an empirical correlation. The local Nusselt number results were compared with available literature and show similar trend and satisfactory agreement.     

Bounds on natural convective heat transfer in a porous layer with fixed heat flux

Using the background field variational method, bounds on natural convective heat transfer in a porous layer heated from below with fixed heat flux are derived from the primitive equations. The enhancement of heat transfer beyond the minimal conduction value (the Nusselt number Nu) is bounded in terms of the non-dimensional forcing scale set by the ‘effective’ Rayleigh number (Rˆ) according to Nu ≤ 0.354Rˆ^1/2 and in terms of the conventional Rayleigh number (Ra) defined by the temperature drop across the layer according to Nu ≤ 0.125Ra. It is presented that fixing the heat flux at the boundaries does not change the linear dependence between Nusselt number and Rayleigh number at high Rayleigh number region.     

Simulation of heat transfer to separation Air flow in a concentric pipe

Flow separations occur in various engineering applications. Computational simulation by using standard k-ε turbulence model was performed to investigate numerically the characteristic of backward-facing step flow in a concentric configuration. This research is focused on the variation of Reynolds number, heat flux and step height in a fully developed turbulent air flow. The design consists of entrance tube, and inner and outer tubes at the test section. The inner tube is placed along the entrance tube at the test section with an outer tube to form annular conduit. The entrance tube diameter was varied to create step height, s of 18.5 mm. The Reynolds number was set between 17,050 and 44,545 and heat flux was set between 719 W/m² and 2098 W/m² respectively. It is observed that the higher Reynolds number with step flow contributes to the enhancement of heat transfer. The reattachment point for q = 719 W/m² is observed at 0.542 m, which is the minimum surface temperature. The experimental data shows slightly lower distribution of surface temperature compared to simulation data. As for the same case in experimental result, the minimum surface temperature is obtained at 0.55 m. The difference between numerical and experimental result is 0.008 m. Finally, it can be inferred that utilizing the computational fluid dynamic package software, agreeable results could be obtained for the present research.     

Experimental study of thermal behaviors in a rectangular channel with baffle of pores

This experimental study attempts to explore the local heat transfer in rectangular channel with baffles, and analyzes the experimental results of baffles with different heights and pores in the event of five Reynolds numbers and three heating quantities. Apart from increasing the perturbation of flow field, the channel's flow field with baffles, which is similar to a backward-facing step flow field, is very helpful to heat transfer. To obtain an optimized baffle and increase the perturbation of flow field, this experiment employed baffles with five heights (H = 10–50 mm) and different numbers of pores (N = 1–3), as well as heat flux: Q = 40–100 l/min, Reynolds number: 702–1752, and heating quantity: q_in = 90–750 W/m². In addition to measurement of overall temperature distribution, emphasis is also placed on analysis of local heat transfer coefficient. Furthermore, heat transfer distribution of channel can be applied to explain how the baffles of pores have an influence upon backward-facing step flow field, shear layer, recirculation region, reattachment region and redeveloped boundary layer. Finally, some empirical formulas derived form experimental results may provide a reference for future design.     

Flow pattern and heat transfer characteristics of R-134a refrigerant during flow boiling in a horizontal circular mini-channel

Flow boiling heat transfer of R-134a refrigerant in a circular mini-channel, 600 mm long with a diameter of 1.75 mm, is investigated experimentally in this study. The test section is a stainless steel tube placed horizontally. Flow pattern and heat transfer coefficient data are obtained for a mass flux range of 200–1000 kg/m² s, a heat flux range of 1–83 kW/m² and saturation pressures of 8, 10, and 13 bar. Five different flow patterns including slug flow, throat-annular flow, churn flow, annular flow and annular-rivulet flow are observed and the heat transfer coefficient data for different flow patterns are presented. The heat transfer coefficient increases with increasing heat flux but is mostly independent of mass flux and vapour quality. In addition, it is indicated from the experiments that the higher the saturation pressure, the lower is the heat transfer coefficient. Comparisons of the present data with the existing correlations are also presented.     

Experimental study of R-134a evaporation heat transfer in a narrow annular duct

An experiment is carried out here to investigate the evaporation heat transfer and associated evaporating flow pattern for refrigerant R-134a flowing in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm. In the experiment, the effects of the duct gap, refrigerant vapor quality, mass flux and saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h_r are examined in detail. For the duct gap of 2.0 mm, the refrigerant mass flux G is varied from 300 to 500 kg/m² s, imposed heat flux q from 5 to 15 kW/m², vapor quality x_m from 0.05 to 0.95, and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the gap of 1.0 mm, G is varied from 500 to 700 kg/m² s with the other parameters varied in the same ranges as that for δ = 2.0 mm. The experimental data clearly show that the evaporation heat transfer coefficient increases almost linearly with the vapor quality of the refrigerant and the increase is more significant at a higher G. Besides, the evaporation heat transfer coefficient also rises substantially at increasing q. Moreover, a significant increase in the evaporation heat transfer coefficient results for a rise in T_sat, but the effects are less pronounced in the narrower duct at a low imposed heat flux and a high refrigerant mass flux. Furthermore, the evaporation heat transfer coefficient increases substantially with the refrigerant mass flux except at low vapor quality. We also note that reducing the duct gap causes a significant increase in h_r. In addition to the heat transfer data, photos of R-134a evaporating flow taken from the duct side show the change of the dominant two-phase flow pattern in the duct with the experimental parameters. Finally, an empirical correlation for the present measured heat transfer coefficient for the R-134a evaporation in the narrow annular ducts is proposed.