ISSN:0145-7632    

DOI:10.1080/01457639708939898

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This study experimentally quantified the change in heat transfer and pressure drop associated with tilting a compact brazed plate heat exchanger from the intended vertical position. Both clockwise and counterclockwise rotations within a plane perpendicular to the fittings were examined. A SWEP B15 × 36 was tested as an R-22 evaporator and condenser under fixed refrigerant state conditions suitable to high-efficiency water-source heat pumps. This study showed that a substantial performance penalty occurred when the evaporator was rotated past 30° from the vertical. The evaporator heat transfer in the horizontal position was 60-75% of the vertical value. For a rotation angle of 30°, the degraded heat transfer was within 5% of the vertical value. Rotation direction and entering refrigerant state had little effect on the performance of the evaporator for rotation angles less than 60”. Only when the evaporator was rotated to the horizontal position did rotation direction and refrigerant slate have much effect. At the horizontal position, a subcooled-entering refrigerant and a counterclockwise rotation both tended to lessen the evaporator heat transfer degradation. Rotation of the condenser to the horizontal position improved the overall heat transfer coefficient by approximately 17-30%. Rotation direction had a negligible effect on the performance of the condenser for angles less than 60°. Both the evaporator and condenser pressure drops were influenced by flow distribution changes as the heat exchangers were rotated.

ISSN:0145-7632    

DOI:10.1080/01457639708939899

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Accurate data have been obtained on the thermal conductivity of six grades of manufactured graphite. In particular, the effect of orientation, manufacturing process, and impregnant content on the thermal conductivity was investigated. Using the thermal conductivity data, a three-dimensional finite-element model has been set up to determine the wall resistance of a particular graphite block. The effect of the anisotropy of the extruded graphite on the wall resistance was investigated. It was found that the anisotropy can be neglected if the wall resistance is calculated by assuming that the blocks have an isotropic thermal conductivity equal to the thermal conductivity perpendicular to the direction of extrusion. Also, the phenol formaldehyde film on the surface of the heat exchanger channels was determined to have a significant thermal resistance. It was calculated that, for typical operating film coefficients with water on both sides, the overall heat transfer coefficient can be expected to increase by more than 50% if the resin layer is removed.

ISSN:0145-7632    

DOI:10.1080/01457639708939900

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

An analysis of the thermal response of a finned-tube, liquid-to-gas cross-flow heat exchanger due to a step change in the liquid inlet temperature is performed. Closed-form solutions for the liquid and gas temperatures as functions of space and time are obtained via the Laplace transform technique for both small and large arguments of the modified Bessel function of the first kind. Using four physically important dimensionless parameters, the response of the liquid and average gas outlet temperatures are studied and presented in the time domain. The analysts is extended to a single-row serpentine coil geometry by accounting for U-tube bends. Using a typical heat exchanger geometry, the effects of the tube bends are shown to be significant. Relevant applications include automotive and HVAC heat exchangers and systems.

ISSN:0145-7632    

DOI:10.1080/01457639708939901

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Pool boiling data for binary mixtures of R-22 /R-124 on plain tubes are reported at reduced pressures of 0.1, 0.15, and 0.2. Significant reductions of heat transfer coefficients for mixtures are found as compared to pure refrigerant, and the reduction of heat transfer coefficient is especially pronounced in the vicinity of the pure component. This phenomenon is strongly related to the nonlinear variation of physical properties of liquid viscosity and latent heat of the mixtures. The available empirical and semiempirical correlations developed for mixtures are compared with the present data. In addition, a correlation based on the present R-22 / R-124 experimental data is proposed. This correlation not only predicts the deterioration of heat transfer coefficients for the present data with success but also predicts the experimental data from other investigators with reasonably good accuracy.

ISSN:0145-7632    

DOI:10.1080/01457639708939902

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The objectives of this article are to propose critical heat flux (CHF) and minimum heat flux (MHF) correlations in transient forced-convective boiling systems and to investigate the parametric effects on CHF and MHF. Experiments were conducted to obtain the CHF and MHF values in forced-convective boiling systems with vertical, parallel, rectangular channels between parallel plates. A nondimensional transient parameter,T=(Z2/αfTsat)(dtw/dt)CHFwas adopted to quantify the transient effects on CHF and MHF values. Experimental ranges are 718.0-2162.0 kg / m2s for mass flux, 7.5-20.3°C for inlet subcooling, and 3.0-8.0 mm for channel gap distance. The system pressure is l.Oatm. The results show that CHF and MHF increased with increasing inlet subcooling. It was also found that CHF and MHF increased somewhat linearly with increasing mass flux at the top of the test section, while they were weakly dependent on the mass flux at the bottom of the test section. New CHF and MHF correlations were developed for upward flow in transient boiling heal transfer with maximum error bands of ±18% for CHF and ±30% for MHF.

ISSN:0145-7632    

DOI:10.1080/01457639708939903

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor