A model for the condensation heat transfer of binary refrigerant mixtures

In the present work turbulent film condensation of nonazeotropic binary mixtures inside a horizontal tube is studied theoretically. The combined heat and mass transfer involved is analyzed through an integral formulation of the continuity, momentum, energy and diffusion equations. As the mass velocity of refrigerant mixtures increases, the condensation heat transfer coefficient increases. The heat transfer coefficient becomes smaller at higher mass quality. As the mole fraction of the more volatile component in binary mixtures increases, the back-diffusion mass flux of the more volatile component reduces in the vapor. As a result the condensation heat transfer coefficient improves with the increase of the inlet mole fraction of the more volatile component especially in the upstream of condenser. The results of the present study show good agreement with the experimental data available.     

An experimental investigation of heat transfer in forced convective boiling of R134a,R123 and R134a/R123 in a horizontal tube

This paper reports an experimental study on flow boiling of pure refrigerants R134a and R123 and their mixtures in a uniformly heated horizontal tube. The flow pattern was observed through tubular sight glasses with an internal diameter of 10 mm located at the inlet and outlet of the test section. Tests were run at a pressure of 0.6 MPa in the heat flux ranges of 5–50 kW/m², vapor quality 0–100 percent and mass velocity of 150–600 kg/m²s. Both in the nucleate boiling-dominant region at low quality and in the two-phase convective evaporation region at higher quality where nucleation is supposed to be fully suppressed, the heat transfer coefficient for the mixture was lower than that for an equivalent pure component with the same physical properties as the mixture. The reduction of the heat transfer coefficient in mixture is explained by such mechanisms as mass transfer resistance and non-linear variation in physical properties etc. In this study, the contribution of convective evaporation, which is obtained for pure refrigerants under the suppression of nucleate boiling, is multiplied by the composition factor by Singal et al. (1984). On the basis of Chen’s superposition model, a new correlation is presented for heat transfer coefficients of mixture.     

Laminar forced convective heat transfer to near-critical water in a tube

Numerical modeling is carried out to investigate forced convective heat transfer to nearcritical water in developing laminar flow through a circular tube. Due to large variations of thermo-physical properties such as density, specific heat, viscosity, and thermal conductivity near thermodynamic critical point, heat transfer characteristics show quite different behavior compared with pure forced convection. With flow acceleration along the tube unusual behavior of heat transfer coefficient and friction factor occurs when the fluid enthalpy passes through pseudocritical point of pressure in the tube. There is also a transition behavior from liquid-like phase to gas-like phase in the developing region. Numerical results with constant heat flux boundary conditions are obtained for reduced pressures from 1.09 to 1.99. Graphical results for velocity, temperature, and heat transfer coefficient with Stanton number are presented and analyzed.     

Performance of heat transfer and pressure drop in a spirally indented tube

In an effort to develop a heat transfer enhancement technique for low temperature applications such as utilization of LNG cold energy, an experiment was carried out to evaluate the heat transfer and the pressure drop performance for a spirally indented tube using ethylene-glycol and water solutions and pure water under horizontal single-phase conditions. The test tube diameter was 14.86 mm and the tube length was 5.38 m. Heat transfer coefficients and friction factors for both inner and outer surfaces of the test tube were calculated from measurements of temperatures, flowrates and pressure drops. Correlations of heat transfer coefficients in the spirally indented tube, which were applicable for laminar and turbulent regimes were proposed for inner, and outer surfaces. The correlations showed that heat transfer coefficients for the spirally indented tube were much higher than those for smooth tubes, increased by more than 8 times depending upon the Reynolds number. The correlations were compared with other correlations for various types of surface roughness. The effect of the Prandtl number on the heat transfer characteristics was discussed. The critical Reynolds number from the laminar flow to the turbulent flow inside the spirally indented tube was found to be around Re=1,000.     

Characteristics of fluid flow and heat transfer in a fluidized heat exchanger with circulating solid particles

The commercial viability of heat exchanger is mainly dependent on its long-term fouling characteristic because the fouling increases the pressure loss and degrades the thermal performance of a heat exchanger. An experimental study was performed to investigate the characteristics of fluid flow and heat transfer in a fluidized bed heat exchanger with circulating various solid particles. The present work showed that the higher densities of particles had higher drag force coefficients, and the increases in heat transfer were in the order of sand, copper, steel, aluminum, and glass below Reynolds number of 5,000.     

Numerical simulation of fully developed flow and heat transfer characteristics in a curved tube with pulsating pressure gradient

Characteristics of fluid flow and convective heat transfer of a pulsating flow in a curved tube have been investigated numerically. The tube wall is assumed to be maintained at a uniform temperature peripherally in a fully developed pulsating flow region. The temperature and flow distributions over a cross-section of a curved tube with the associated velocity field need to be studied in detail. This problem is of particular interest in the design of Stirling engine heat exchangers and in understanding the blood flow in the aorta. The time-dependent, elliptic governing equations are solved, employing finite volume technique. The periodic steady state results are obtained for various governing dimensionless parameters, such as Womersley number, pulsation amplitude ration, curvature ratio and Reynolds number. The numerical results indicate that the phase difference between the pressure gradient and averaged axial velocity increases gradually up to π/2 as Womersley number increases. However, this phase difference is almost independent of the amplitude ratio of pulsation. It is also found that the secondary flow patterns are strongly affected by the curvature ratio and Reynolds number. These, in turn, give a strong influence on the convective heat transfer from the pipe wall to the pulsating flow. The results obtained lead to a better understanding of the underlying physical process and also provide input that may be used to design the relevant system. The numerical approach is discussed in detail, and the aspects that must be included for an accurate simulation are discussed.     

Flow and heat transfer measurements of film injectant from a row of holes with compound angle orientations

An experiment has been conducted on the flow and heat transfer characteristics of film coolant injected from a row of five holes with compound angle orientations of 35° inclination angle and 45° orientation angle. The Reynolds number number based on the mainstream velocity and injection hole diameter 3.58 x10⁴. Three-dimensional velocity, film cooling effectiveness and heat transfer coefficient data are presented at three different mass flux ratios of 0.5, 1.0 and 2.0. Flow entrainment has been found between the vortices generated by adjacent injectants. The injectant with compound angle orientation entrains not only the mainstream boundary layer flow but also the adjacent injectant. Because of the flow entrainment, the injectant. With compound angle orientation is characterized by a single vortex while two bound vortices are usually observed in the case of simple angle injection. The strength of the secondary flow depends strongly on the mass flux ratio, which shows significant influence on the film cooling effectiveness and heat transfer coefficient.     

Evaporation heat transfer and pressure drop characteristics of r-134a in the oblong shell and plate heat exchanger

The evaporation heat transfer coefficienthr and frictional pressure drop δp_f of refrigerant R-134a flowing in the oblong shell and plate heat exchanger were investigated experimentally in this study. Four vertical counterflow channels were formed in the oblong shell and plate heat exchanger by four plates of geometry with a corrugated sinusoid shape of a 45° chevron angle. Upflow of refrigerant R-134a boils in two channels receiving heat from downflow of hot water in other channels. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation temperature and vapor quality of R- 134a were explored in detail. Similar to the case of a plate heat exchanger, even at a very low Reynolds number, the flow in the oblong shell and plate heat exchanger remains turbulent. The results indicate that the evaporation heat transfer coefficienthr and pressure drop Δp_f increase with the vapor quality. A rise in the refrigerant mass flux causes an increase in theh _r and Δp_f. But the effect of the average heat flux does not show significant effect on the h_r and Δp_f. Finally, at a higher saturation temperature, both theh _r and Δp_f are found to be lower. The empirical correlations are also provided for the measured heat transfer coefficient and pressure drop in terms of the Nusselt number and friction factor.     

Turbulent flow and heat transfer characteristics of a two-dimensional oblique plate impinging jet

Turbulent flow and heat transfer characteristics of a two-dimensional oblique plate impinging jet (OPIJ) were experimentally investigated. The local heat transfer coefficients were measured using thermochromic liquid crystals. The jet mean velocity and turbulent intensity profiles were also measured along the plate. The jet Reynolds number (Re, based on the nozzle width) ranged from 10, 000 to 35,000, the nozzle-to-plate distance (H/B) from 2 to 16, and the oblique angle (α) from 60 to 90 degree. It has been found that the stagnation point shifted toward the minor flow region as the oblique angle decreased and the position of the stagnation point nearly coincided with that of the maximum turbulent intensity. It has also been observed that the local Nusselt numbers in the minor flow region were larger than those in the major flow region for the same distance along the plate mainly due to the higher levels in turbulent intensity caused by more active mixing of the jet flow.     

Effects of combustor-level high inlet turbulence on the endwall flow and heat/mass transfer of a high-turning turbine rotor cascade

Experimental data are presented which describe the effects of a combustor-level high free-stream turbulence on the near-wall flow structure and heat/mass transfer on the endwall of a linear high-turning turbine rotor cascade. The endwall flow structure is visualized by employing the partial- and total-coverage oil-film technique, and heat/mass transfer rate is measured by the naphthalene sublimation method. A turbulence generator is designed to provide a highly-turbulent flow which has free-stream turbulence intensity and integral length scale of 14.7% and 80mm, respectively, at the cascade entrance. The surface flow visualizations show that the high free-stream turbulence has little effect on the attachment line, but alters the separation line noticeably. Under high free-stream turbulence, the incoming near-wall flow upstream of the adjacent separation lines collides more obliquely with the suction surface. A weaker lift-up force arising from this more oblique collision results in the narrower suction-side corner vortex area in the high turbulence case. The high free-stream turbulence enhances the heat/mass transfer in the central area of the turbine passage, but only a slight augmentation is found in the endwall regions adjacent to the leading and trailing edges. Therefore, the high free-stream turbulence makes the endwall heat load more uniform. It is also observed that the heat/mass transfers along the locus of the pressure-side leg of the leading-edge horseshoe vortex and along the suctionside corner are influenced most strongly by the high free-stream turbulence. In this study, the endwall surface is classified into seven different regions based on the local heat/mass transfer distribution, and the effects of the high free-stream turbulence on the local heat/mass transfer in each region are discussed in detail.