Richard N. Christensen

A condensation heat transfer correlation for millimeter-scale tubing with flow regime transition

This study documents local convection heat transfer and flow regime measurements for HFC-134a condensing inside a horizontal rectangular multi-port aluminum condenser tube of 1.46 mm hydraulic diameter. The data is compared with condensation heat transfer correlations and flow regime maps from the literature. Existing correlations are found to overpredict both heat transfer and the stratified-to-annular flow regime transition velocity. Results of the experiments suggest that liquid drawn into the corners of the tube alter the phase distribution in the annular flow regime as well as stabilizing the annular flow regime at lower vapor velocities. To predict the heat transfer data, two correlations, each representing the physics of the specific phase distributions, are developed. A boundary layer analysis is applied for annular flow, in which the friction multiplier and dimensionless boundary layer temperature are evaluated specifically for this tube configuration. For stratified flow, documented film condensation and single-phase forced convection correlations are combined with straightforward void fraction weighting. Finally, a weighting correlation is successfully proposed to account for the all data regardless of the mix of flow regimes experienced. This weighting applies the result of a modified flow regime map developed from the flow visualizations. The final result is a practical correlation for the design of a condenser with millimeter-scale tubes.

FREE CONVECTION IN A VERTICAL ANNULUS WITH CONSTANT HEAT FLUX ON THE INNER WALL.

Heat transfer measurements are presented for free convection in a vertical annulus wherein the inner cylinder is at constant surface heat flux and the outer cylinder is at constant temperatuare. Overall heat transfer data re corrected for thermal radiation in the annulus. Rayleigh numbers span the conduction, transition and boundary layer regimes of flow, and average heat transfer coefficients are obtained with air and helium as the working fluids. The range of Rayleigh number is 10/sup 3/

Enhanced heat transfer of drag reducing surfactant solutions with fluted tube-in-tube heat exchanger

Abstract Solutions containing drag reducing additives also show reduced heat transfer which limits their use in hydronic cooling and heating systems where heat exchange is critical. For Reynolds numbers 10,000–50,000 and test fluid inlet temperatures 50–70°C, a fluted inner tube heat exchanger showed increased heat transfer coefficients for both cationic and zwitterionic/anionic drag reducing surfactant solutions. The pressure drop penalty for heat transfer enhancement of the cationic surfactant solution flowing through the fluted tube is high while for the zwitterionic/anionic solution, significant heat transfer improvement was achieved with only a modest pressure drop penalty.

Enhancing heat transfer ability of drag reducing surfactant solutions with static mixers and honeycombs

Abstract Solutions containing drag reducing additives also show reduced heat transfer which limits their use in district heating and cooling recirculation systems where heat exchange is critical. In this study, static mixers A and B and honeycombs were installed at the entrance to a heat exchanger to break the solution microstructure temporarily and thereby enhancing their heat transfer ability when passing through the heat exchanger. The effectiveness of the destructive devices in enhancing the heat transfer ability of drag reducing cationic and mixed zwitterionic/anionic surfactant solutions was investigated together with the microstructure recovery time and pressure drop penalty paid for the heat transfer enhancement.

Performance evaluation of a generator-absorber heat-exchange heat pump

A generator-absorber heat-exchange (GAX) heat pump was modeled in this study using OSU-ABSIM, a modular steady-state simulation program. While the thermodynamics of GAX cycles have been investigated in the past, a comprehensive implementation-related performance evaluation of this cycle is not available. Therefore, in the present study, the complete absorption system was analyzed for the heating and cooling modes, including air-to-hydronic heat exchangers and a natural gas-fired generator. Several significant variables that affect cycle performance were systematically investigated. The effect of ambient temperature on system COP was determined for the cooling and heating modes. The system cooling COP at the rating point was maximized by varying the heat-exchanger UA values. The decrease in GAX overlap at low ambients, and the corresponding transformation to the absorber heat-exchange cycle was also modeled; and the implications on system COP were investigated. Furthermore, the role of an additional solution-solution heat exchanger at the low ambients in enhancing COP was quantified. The results of this study can be used to determine the merits of this cycle versus other absorption heat-pump alternatives. The comprehensive investigation of performance (including off-design conditions) allows the evaluation of year-round performance, which can be used in conjunction with climate data to establish the applicability to different geographic locations.

Experimental Investigation of Heat Transfer in Coiled Annular Ducts

Forced convection heat transfer in coiled annular ducts was investigated experimentally. Average heat transfer coefficients were obtained for both laminar and transition flows. Two coiling diameters and two annulus radius ratios were used in the study. The data were correlated with Dean number and Reynolds number separately and compared with the available studies of coiled circular tubes and straight annular ducts. It was found that coiling augments the heat transfer coefficients above the values for a straight annulus especially in the laminar region. However, the augmentation is less than would be expected for a coiled circular tube. The augmentation decreases as the flow enters the transition region.

Experimental Investigation of Free Convection in a Vertical Rod Bundle—A General Correlation for Nusselt Numbers

Free convection in two vertical, enclosed rod bundles has been experimentally investigated for a wide range of Rayleigh numbers. A uniform power dissipation per unit length is supplied to each rod, and the enclosing outer cylinder is maintained at constant temperature. Nusselt numbers for each rod, as well as an overall value for each bundle, have been obtained as a function of Rayleigh number. Comparison of the results for air and water as the working fluid indicate that, for a fixed Rayleigh number, an increase in the Prandtl number produces a reduction in the Nusselt number. This is contrary to what has been reported for vertical cavities and is attributed to curvature effects. Furthermore, the data reveal the interesting fact that it is quite possible for the individual rods in the bundle to exchange energy with the working fluid via different but coexisting regimes at a given power dissipation. Also, as the Rayleigh number is increased, the rods each tend to assume nearly the same heat transfer coefficient. Finally, a correlation for the overall convective Nusselt number is developed in terms of Rayleigh number and geometric parameters.

Heat Transfer and Pressure Drop Characteristics of Spirally Fluted Annuli: Part II—Heat Transfer

We present results of a comprehensive study of heat transfer and pressure drop in annuli with spirally fluted inner tubes for the laminar, transition, and turbulent flow regimes. Fourteen fluted tubes with varying geometries were studied, with up to three outer smooth tubes for each fluted tube. Flow patterns and transitions between flow regimes investigated through visualization tests, friction factor data (from Part I), and tube surface-temperature measurements were used to explain the enhancement phenomena. The fluted inner tubes induced a significant degree of swirl in the flow, and transition occurred in the 310<Re<1000 range. A Nusselt number correlation was developed in terms of the fluted annulus friction factor developed in Part I and geometric parameters. Nusselt numbers were between 4 and 20 times the smooth annulus values in the low Re range, while turbulent enhancements with between 1.1 and 4.0. The enhancement values can be used in conjunction with friction factor increase values reported in Part I to determine appropriate ranges of applicability for spirally enhanced annuli

Experimental investigation of heat transfer and pressure drop characteristics of flow through spirally fluted tubes

Spirally fluted tubes are used extensively in the design of tubular heat exchangers. In previous investigations, results for tubes with flute depths e/Dvi < 0.2 were reported, with most correlations applicable for Re ≥ 5000. This paper presents the results of an experimental investigation of the heat transfer and pressure drop characteristics of spirally fluted tubes with the following tube and flow parameter ranges: flute depth e/Dvi = 0.1−0.4, flute pitch p/Dvi = 0.4−7.3, helix angle θ/90° = 0.3−0.65, Re = 500−80,000, and Pr = 2−7. The heat transfer coefficients inside the fluted tube were obtained from measured values of the overall heat transfer coefficient using a nonlinear regression scheme. The friction factor data obtained consisted of 507 data points. The proposed correlation for the friction factor predicts 96% of the database within ±20%. The heat transfer correlation for the range 500 ≤ Re ≤ 5000 predicts 76% of the database (178 data points) within ±20%, and the correlation for the higher Re range predicts 97% of the 342 data points within ±20%. Comparison of heat transfer and friction data show that these tubes are most effective in the laminar and transition flow regimes. The present results show that the increase of flute depth in the range considered does not improve heat transfer.

An improved scheme for determining heat transfer correlations from heat exchanger regression models with three unknowns

Abstract A nonlinear regression scheme for determining heat transfer correlations from overall heat exchanger measurements is presented. The proposed scheme is valid for any two-fluid heat exchanger data where only one fluids thermal resistance varies. The resulting regression model has three unknowns; two are associated with the variable resistance, and the third is the sum of all other resistances. The model can be analyzed only by using nonlinear regression because it cannot be transformed into linear form. Unlike existing approximate methods of analysis (known as modified Wilson plots), the proposed scheme is guaranteed to converge if a solution exists. The scheme was successfully tested by analyzing data collected on a tube-fin heat exchanger with continuous plain fins.