Ralph L. Webb

Performance evaluation criteria for use of enhanced heat transfer surfaces in heat exchanger design

Abstract This study extends previous work of Bergles and Webb to establish a broad range of Performance Evaluation Criteria (PEC) applicable to single phase flow in tubes. The equations include the effects of shellside enhancement and fouling and are applicable to roughness and internally finned tubes. Detailed procedures are outlined to calculate the performance improvement and to select the ‘optimum’ surface geometry. PEC are presented for four design cases:(1) reduced heat exchanger material; (2) increased heat duty; (3) reduced log-mean temperature difference; and (4) reduced pumping power. The 11 cases discussed include fixed flow area and variable flow area. Appropriate PEC for two-phase exchangers area also discussed.

Forced convection heat transfer in helically rib-roughened tubes

Abstract This work presents experimental information for single-phase forced convection in a circular tube containing a two-dimensional rib roughness. It extends the state-of-the-art by examining the effect of the rib helix angle. Although prior studies have proposed that helix angles less than 90° will provide superior heat transfer per unit pumping power, no data have been reported for flow in circular tubes. The present work reports the heat transfer and friction characteristics for air flow with three helix angles (30, 49 and 70°) all having a rib pitch-to-height ratio of 15. The preferred helix angle is approximately 45°. The data are correlated in a form to permit performance prediction with any relative roughness size ( e D ). The benefits of the roughness for heat exchanger applications are quantitatively established.

Correlation of two-phase friction for refrigerants in small-diameter tubes

Single-phase and adiabatic two-phase flow pressure drop were measured for R-134A, R-22 and R-404A flowing in a multi-port extruded aluminum tube with hydraulic diameter of 2.13 mm, and in two copper tubes having inside diameters of 6.25 and 3.25 mm, respectively. The single-phase friction factor was predicted within ±10% using the Blasius friction correlation. The Friedel correlation did not predict the two-phase data accurately, especially for high reduced pressure. Using the data taken in the present and in a previous study, a new correlation for two-phase friction pressure drop in small diameter tubes was developed by modifying the Friedel correlation. The new correlation predicts 119 data points with a mean deviation of 11.5%.

Heat transfer and friction in the offset stripfin heat exchanger

This paper presents analytical models to predict the heat transfer coefficient and the friction factor of the offset strip-fin heat exchanger surface geometry. Two flow regimes are defined—laminar and turbulent. Based on the conditions in the wake, an equation is developed to predict transition from laminar to turbulent flow. Flow visualization experiments were performed to identify the flow structure at transition. The condition predicted by the transition equation corresponds to onset of oscillating velocities in the fin wakes. Equations are developed for the Nusselt number and friction factor by writing energy and momentum balances on a unit cell of the offset strip-fin geometry. A numerical solution is used to calculate Nu and f on the fins in the laminar regime, and a semi-empirical approach is used for the turbulent regime. Predictions are compared to data on scaled-up geometries, taken in the present study, and data on actual heat exchangers. The models predict all data within ±20%.

The Evolution of Enhanced Surface Geometries for Nucleate Boiling

This paper surveys the evolution of special surface geometries that promote high-performance nucleate boiling. Early work by Jakob and Fritz in 1931 showed that emery paper roughening or machined grooves provided only temporary performance increase. However, this improvement dissipated after a few days to the flat surface value. There was little sustained interest in this unique, but apparently unuseful, phenomenon until the mid-1950s. During the period 1955-1965, supporting fundamental studies of the character and stability of nucleation sites provided a basis for renewed efforts to develop a high area density of stable, artificially formed nucleation sites whose performance does not deteriorate with time. Beginning in 1968 industrial research produced patented technology that achieved the long-sought goal. In 1980 at least six high-performance nucleate boiling surfaces were commercially available. The technology reported in this paper represents a dramatic advance in the field of heat transfer.

Nucleate pool boiling data for five refrigerants on plain, integral-fin and enhanced tube geometries

Abstract Data are presented for nucleate pool boiling on five different horizontal tube geometries using five refrigerants at two saturation temperatures. The refrigerants tested are R-11, R-12, R-22, R-123 and R-134a at saturation temperatures of 4.44°C (40°F) and 26.7°C (80°F). The tube geometries tested are a plain tube, a 1024 fins/m integral-fin tube, and three commercially used enhanced tube geometries (GEWA TX19, GEWA SE, and Turbo-B). The wide range of data reported here is new to the literature. The ability of the Cooper and the Stephan-Abdelsalam correlations to predict the plain tube data is evaluated. The slopes of the boiling coefficient vs heat flux curve are found to be heat flux dependent. Of particular importance are the data on R-123 and R-134a, which are intended to replace R-11 and R-12, respectively. Except for the Turbo-B with R-11, it appears that the boiling coefficients for R-123 and R-134a are within 10% of the values for R-11 and R-12, respectively.

Condensation of R-12 in small hydraulic diameter extruded aluminum tubes with and without micro-fins

This manuscript provides heat transfer data for R-12 condensation and subcooled liquid in small hydraulic diameter, flat extruded aluminum tubes. The tube outside dimensions are 16 mm × 3 mm (high) × 0.5 rrm (wall thickness). The tubes contain three internal membranes, which separate the flow into four parallel channels. Two internal geometries were tested: one had a plain inner surface and the other had micro-fins, 0.2 mm high. Data are presented for the following range of variables: vapor qualities (12–97%), mass velocity (400–1400 kg s−1 m−2), and heat flux (4–12 kW m−2). The overall heat transfer coefficient was measured for water-to-refrigerant heat transfer, and the modified Wilson plot method used to determine the heat transfer coefficient for water-side flow in the annulus. Then, the tube-side condensation coefficient was extracted from the measured UA-value. The data show that the condensation coefficient increases with heat flux to the 0.20 power. The subcooled heat transfer coefficient for both geometries is well predicted using the Petukhov equation with hydraulic diameter. At low mass velocity, the Akers correlation agrees well with the plain tube data, and overpredicts the data 10–20% at high mass velocity. The micro-fin tube shows significantly higher performance than predicted by the Akers correlation (based on hydraulic diameter) for vapor qualities greater than 0.5. The authors propose that surface tension force is effective in enhancing the condensation coefficient for vapor quality greater than 0.5. The proposed surface tension enhancement is particularly strong at the lower mass velocities.

Calculation of Fin Efficiency for Wet and Dry Fins

This paper presents a quantitative evaluation of methods used to calculate fin efficiency for dry and wet fins. The exact solution for circular fins uses Bessel functions, which are tedious to evaluate. The authors provide an empirical modification to the Schmidt equation (Hong and Webb equation), which provides improved accuracy for circular fins. Existing methods to evaluate the fin efficiency of rectangular, plain plate fins are evaluated. The plate-fin methods are compared against the fin efficiency calculated by the sector method using Bessel functions. Use of the sector method with the Hong and Webb equation is recommended. Cases for which the simple “equivalent circular fin” method is a very good approximation are defined. Methods to calculate the fin efficiency of enhanced fin geometries are discussed. The authors show that the wet surface fin efficiency for any fin geometry may be calculated using a simple modification of the dry surface fin efficiency equation. Example calculations are presented...

A fundamental understanding of factors affecting frost nucleation

Abstract Theoretical analysis of the nucleation process for frost formation on a cold surface shows that the air at the cold surface should be supersaturated in order for frost nucleation to occur. This understanding is new, relative to previously published frost growth models. Further, the supersaturation degree is dependent on the surface energy, which is related to the water contact angle. The theoretical predictions were compared to experimental results, and reasonable agreement was obtained. Qualitatively, a low energy surface (high contact angle) requires higher supersaturation degree for frost nucleation than a high energy surface. Quantitatively, the experimental data shows that the low energy surface requires approximately 10 times higher supersaturation degree than the high energy surface when the contact angle difference is approximately 80° at −20 °C surface temperature. The factors affecting the surface energy such energy such as temperature, surface roughness, and foreign particles are discussed in this paper.

Data reduction for air-side performance of fin-and-tube heat exchangers

Abstract The present study focuses on the data reduction method to obtain the air-side performance of fin-and-tube heat exchangers. The data reduction methodology for air-side heat transfer coefficients in the literature is not based on a consistent approach. This paper recommends standard procedures for dry surface heat transfer in finned-tube heat exchangers having water on the tube-side. Inconsistencies addressed include the e-NTU relationships, calculation of the tube-side heat transfer coefficient, calculation of fin efficiency, and whether entrance and exit loss should be included in the reduction of friction factors. Use of the recommended standardized methodology will provide more meaningful data for use in the development of correlations, or for performance comparison purposes.