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1.
This paper investigates the effect of vapour super-heating on hydrocarbon refrigerant 600a (Isobutane), 290 (Propane) and 1270 (Propylene) condensation inside a brazed plate heat exchanger.Vapour super-heating increases heat transfer coefficient with respect to saturated vapour, whereas no effect was observed on pressure drop.The super-heated vapour condensation data shows the same trend vs. refrigerant mass flux as the saturated vapour condensation data, but with higher absolute values. A transition point between gravity controlled and forced convection condensation has been found for a refrigerant mass flux around 15-18 kg m−2 s−1 depending on refrigerant type. The super-heated vapour heat transfer coefficients are from 5% to 10% higher than those of saturated vapour under the same refrigerant mass flux.The experimental heat transfer coefficients have been compared against Webb (1998) model for forced convection condensation of super-heated vapour: the mean absolute percentage deviation between the experimental and calculated data is ±18.3%.HC-1270 shows super-heated vapour heat transfer coefficient 5% higher than HC-600a and 10-15% higher than HC-290 together with total pressure drops 20-25% lower than HC-290 and 50-66% lower than HC-600a under the same mass flux.  相似文献   

2.
An experimental study was made of the thermal and hydraulic characteristics of a three-phase fluidized bed cooling tower. The experiments were carried out in a packed tower of 200 mm diameter and 2.5 m height. The packing used was spongy rubber balls 12.7 mm in diameter and with a density of 375 kg/m3. The tower characteristic was evaluated. The air-side pressure drop and the minimum fluidization velocity were measured as a function of water/air mass flux ratio (0.4–2), static bed height (300–500 mm), and hot water inlet temperature (301–334 K).

The experimental results indicate that the tower characteristics KaV/L increases with increases in the bed static height and hot water inlet temperature and with decreases in the water/air mass flux ratio. It is also shown that the air-side pressure drop increases very slowly with increases in air velocity. The minimum, fluidization velocity was found to be independent of the static bed height.

The data obtained were used to develop a correlation between the tower characteristics, hot water inlet temperature, static bed height, and the water/air mass flux ratio. The mass transfer coefficient of the three-phase fluidized bed cooling tower is much higher than that of packed-bed cooling towers with higher packing height.  相似文献   


3.
Flow regime transitions due to cavitation in the flow through an orifice   总被引:4,自引:0,他引:4  
This paper presents both experimental and theoretical aspects of the flow regime transitions caused by cavitation when water is passing through an orifice. Cavitation inception marks the transition from single-phase to two-phase bubbly flow; choked cavitation marks the transition from two-phase bubbly flow to two-phase annular jet flow.

It has been found that the inception of cavitation does not necessarily require that the minimum static pressure at the vena contracta downstream of the orifice, be equal to the vapour pressure liquid. In fact, it is well above the vapour pressure at the point of inception. The cavitation number [σ = (P3Pv)/(0.5 pV2); here P3 is the downstream pressure, Pv is the vapour pressure of the liquid, ρ is the density of the liquid and V is the average liquid velocity at the orifice] at inception is independent of the liquid velocity but strongly dependent on the size of the geometry. Choked cavitation occurs when this minimum pressure approaches the vapour pressure. The cavitation number at the choked condition is a function of the ratio of the orifice diameter (d) to the pipe diameter (D) only. When super cavitation occurs, the dimensionless jet length [L/(D - d); where L is the dimensional length of the jet] can be correlated by using the cavitation number. The vaporization rate of the surface of the liquid jet in super cavitation has been evaluated based on the experiments.

Experiments have also been conducted in which air was deliberately introduced at the vena contracta to simulate the flow regime transition at choked cavitation. Correlations have been obtained to calculate the critical air flow rate required to cause the flow regime transition. By drawing an analogy with choked cavitation, where the air flow rate required to cause the transition is zero, the vapour and released gas flow rate can be predicted.  相似文献   


4.
The nonazeotropic binary mixtures such as, methanol/water, ethanol/water and ammonia/water, have variable boiling and dew points, depending on the combination of substance and those mass fraction. It is expected to have a higher performance as a result of decreasing the thermodynamically irreversible loss, when there is a relevant mass fraction. Therefore, ammonia/water mixture is expected to use as working fluid in small temperature difference power generation cycles and absorption refrigeration cycles. However, few experiments were carried out for measuring heat transfer coefficient for ammonia/water mixture in the world. An experimental study has been carried out to measure boiling heat transfer coefficient of an ammonia/water mixture on a horizontal heated surface at low pressure of 0.2, 0.4 and 0.7 MPa and at low mass fraction of 0 < C < 0.27 and at high pressure 0.7, 1.0 and 1.5 MPa and at mass fraction of 0.5 < C < 1.0 and at heat flux under critical heat flux the heat transfer coefficient are compared with existing correlations prediction and a revised correlation can be proposed to predict them well.  相似文献   

5.
A series of tests have been performed to determine the saturated critical heat flux (CHF) in 0.5 and 0.8 mm internal diameter microchannel tubes as a function of refrigerant mass velocity, heated length, saturation temperature and inlet liquid subcooling. The tested refrigerants were R-134a and R-245fa and the heated length of microchannel was varied between 20 and 70 mm. The results show a strong dependence of CHF on mass velocity, heated length and microchannel diameter but no influence of liquid subcooling (2–15 °C) was observed. The experimental results have been compared to the well-known CHF single-channel correlation of Y. Katto and H. Ohno [An improved version of the generalized correlation of critical heat flux for the forced convective boiling in uniformly heated vertical tubes, Int. J. Heat and Mass Transfer 27 (9) (1984) 1641–1648] and the multichannel correlation of W. Qu and I. Mudawar [Measurement and correlation of critical heat flux in two-phase microchannel heat sinks, Int. J. Heat and Mass Transfer 47 (2004) 2045–2059]. The comparison shows that the correlation of Katto–Ohno predicts microchannel data with a mean absolute error of 32.8% with only 41.2% of the data falling within a ±15% error band. The correlation of Qu and Mudawar shows the same trends as the CHF data but significantly overpredicts them. Based on the present experimental data, a new microscale version of the Katto–Ohno correlation for the prediction of CHF during saturated boiling in microchannels has been proposed.  相似文献   

6.
Two-phase friction and heat transfer characteristics for R-22/R-407C inside a 6.5-mm smooth tube are reported in this study. The heat transfer results for G=100 and 400 kg/m2 s were reported in the present study, and the adiabatic frictional pressure drop was recorded in the range of 100 to 700 kg/m2 s. It is found that the development of flow pattern for R-407C falls behind R-22. This may explain the lower pressure drops for R-407C. The major heat transfer mechanism at low mass flux is nucleate boiling, and virtually becomes the convective evaporation as mass flux increase to G=400 krg/m2 s, Meanwhile, the reduction of heat transfer coefficients for R-407C mixtures are especially profound at low mass flux, and the reduction of heat transfer coefficient decreases with the increase of mass flux.  相似文献   

7.
The influence of test section orientation and diameter on flow boiling crisis occurring in tubes has been studied experimentally using Freon-12 as a coolant. At low mass flux the critical heat flux (CHF) was lower in horizontal flow than in vertical. As either the liquid or vapour velocity, or both, were increased the vertical and horizontal CHF results converged. Above a mass flux of 4Mg · m−2 · s−1 the results were essentially identical.

The effect of tube diameter on boiling crisis in general depends crucially on the parameters which are maintained constant when the comparison is made.  相似文献   


8.
This paper presents the heat transfer coefficients and the pressure drop measured during HFC-410A condensation inside a commercial brazed plate heat exchanger: the effects of saturation temperature, refrigerant mass flux and vapour super-heating are investigated. The heat transfer coefficients show weak sensitivity to saturation temperature and great sensitivity to refrigerant mass flux and vapour super-heating. At low refrigerant mass flux (<20 kg/m2 s) the saturated vapour condensation heat transfer coefficients are not dependent on mass flux and are well predicted by Nusselt [W. Nusselt, Die oberflachenkondensation des wasserdampfes, Energy 60 (1916) 541–546, 569–575] analysis for vertical surface: the condensation process is gravity controlled. For higher refrigerant mass flux (>20 kg/m2s) the saturated vapour condensation heat transfer coefficients depend on mass flux and are well predicted by Akers et al. [W.W. Akers, H.A. Deans, O.K. Crosser, Condensing heat transfer within horizontal tubes, Chem. Eng. Prog. Symp. Series 55 (1959) 171–176] equation: forced convection condensation occurs. In the forced convection condensation region the heat transfer coefficients show a 30% increase for a doubling of the refrigerant mass flux. The condensation heat transfer coefficients of super-heated vapour are 8–10% higher than those of saturated vapour and are well predicted by Webb [R.L. Webb, Convective condensation of superheated vapor, ASME J. Heat Transfer 120 (1998) 418–421] model. A simple linear equation based on the kinetic energy per unit volume of the refrigerant flow is proposed for the computation of the frictional pressure drop.  相似文献   

9.
Three dimensional numerical studies were performed for laminar heat transfer and fluid flow characteristics of wavy fin heat exchangers with elliptic/circular tubes by body-fitted coordinates system. The simulation results of circular tube were compared with the experiment data, then circular and elliptic (e = b/a = 0.6) arrangements with the same minimum flow cross-sectional area were compared. A max relative heat transfer gain of up to 30% is observed in the elliptic arrangement, and corresponding friction factor only increased by about 10%. The effects of five factors on wavy fin and elliptic tube heat exchangers were examined: Reynolds number (based on the smaller ellipse axis, 500  4000), eccentricity (b/a, 0.6  1.0), fin pitch (Fp/2b, 0.05  0.4), fin thickness (Ft/2b, 0.006  0.04) and tube spanwise pitch (S1/2b, 1.0  2.0). The results show that with the increasing of Reynolds number and fin thickness, decreasing of the eccentricity and spanwise tube pitch, the heat transfer of the finned tube bank are enhanced with some penalty in pressure drop. There is an optimum fin pitch (Fp/2b = 0.1) for heat transfer, but friction factor always decreases with increase of fin pitch. And when Fp/2b is larger than 0.25, it has little effects on heat transfer and pressure drop. The results were also analyzed from the view point of field synergy principle. It was found that the effects of the five factors on the heat transfer performance can be well described by the field synergy principle.  相似文献   

10.
An experimental study was performed to understand the nucleate boiling heat transfer of water–CuO nanoparticles suspension (nanofluids) at different operating pressures and different nanoparticle mass concentrations. The experimental apparatus is a miniature flat heat pipe (MFHP) with micro-grooved heat transfer surface of its evaporator. The experimental results indicate that the operating pressure has great influence on the nucleate boiling characteristics in the MFHP evaporator. The heat transfer coefficient and the critical heat flux (CHF) of nanofluids increase greatly with decreasing pressure as compared with those of water. The heat transfer coefficient and the CHF of nanofluids can increase about 25% and 50%, respectively, at atmospheric pressure whereas about 100% and 150%, respectively, at the pressure of 7.4 kPa. Nanoparticle mass concentration also has significant influence on the boiling heat transfer and the CHF of nanofluids. The heat transfer coefficient and the CHF increase slowly with the increase of the nanoparticle mass concentration at low concentration conditions. However, when the nanoparticle mass concentration is over 1.0 wt%, the CHF enhancement is close to a constant number and the heat transfer coefficient deteriorates. There exists an optimum mass concentration for nanofluids which corresponds to the maximum heat transfer enhancement and this optimum mass concentration is 1.0 wt% at all test pressures. The experiment confirmed that the boiling heat transfer characteristics of the MFHP evaporator can evidently be strengthened by using water/CuO nanofluids.  相似文献   

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