COMPARISON OF THE EVAPORATION AND CONDENSATION HEAT TRANSFER COEFFICIENTS ON THE OUTSIDE OF SMOOTH , MICRO FIN AND VIPERTEX 1 EHT ENHANCED HEAT TRANSFER TUBES

An experimental investigation was performed to evaluate condensation and evaporation heat transfer on the outside of a smooth tube, herringbone micro fin tube and the Vipertex 1EHT enhanced heat transfer tube as a function of mass flux. Heat transfer enhancement is an important factor in obtaining energy efficiency improvements in two phase heat transfer applications. Utilization of enhanced heat transfer tubes is an effective enhancement method that is utilized in the development of high performance thermal systems. VipertexTM enhanced surfaces have been designed and produced through material surface modifications, creating flow optimized heat transfer tubes that increase heat transfer. Heat transfer processes that involve phase-change processes are typically efficient modes of heat transfer; however current energy demands and the desire to increase efficiencies of systems have prompted the development of enhanced heat transfer surfaces that can be used in processes involving evaporation and condensation. Surface enhancement of the 1EHT tube is accomplished through the use of a primary dimple enhancement coupled with a secondary background pattern made up of petal arrays. Enhancement of the herringbone is accomplished through the use of microfins. Convective condensation heat transfer and pressure loss characteristics were investigated using R410A on the outside of: (i) a smooth tube (outer diameter 12.7 mm); (ii) an external herringbone tube (fin root diameter 12.7 mm); and (iii) the 1EHT tube (outer diameter 12.7 mm) for mass flux ranging from 8 to 50 kg/ (m 2 s); at a saturation temperature of 318 K; with an inlet quality of 0.8 and an outlet quality of 0.1. For these conditions, both the 1EHT tube, and the herringbone tube did not perform as well as the smooth tube. This was an unexpected result. Additionally the study also included a determination of the evaporation heat transfer coefficients using R410A on the outside of the same three tubes. The nominal evaporation temperature was 279 K; for a mass flux that ranged from 10 to 40 kg/m 2 s; with an inlet quality of 0.1 and the outlet quality of 0.8. Excellent heat transfer performance is demonstrated by the 1EHT tube showing an enhancement ratio of approximately 1.4. Evaporation heat transfer coefficient enhancement values for the herringbone tube ranges from 1.5 to 2.2. For the considered conditions, both the herringbone and 1EHT tubes have higher pressure drops than smooth tubes. Microfins, surface roughness and three dimensional enhanced surfaces are often incorporated on the surface of tubes in order to enhance heat transfer performance. Under many conditions, enhanced surface tubes can recover more energy and provide the opportunity to advance the design of many heat transfer products. Enhanced heat transfer tubes are widely used in refrigeration and air-conditioning applications in order to reduce cost and create a smaller application footprint. A new type of enhanced heat transfer tube has been created using dimples/protrusions with secondary petal arrays; therefore it is important to investigate the heat transfer characteristics of the new Vipertex 1EHT enhanced surface tube and compare it to other tubes. INTRODUCTION Recently, the study of enhanced heat transfer has received a good deal of attention due to increased demands by industry for heat exchange equipment that is less expensive to build and operate. Savings in materials and energy use provides a strong motivation for the development of improved methods of heat transfer. Additional motivation is discussed by Reay [1] where he states that “between 1900 and 1955 the average rate of global energy use rose from about 1 TW to 2 TW. From 1955 to 1999 energy use rose from 2 TW to about 12 TW. ” In 2011 energy use rose to an estimated value of 17 TW; and it is currently above 20 TW; with future demand expected to increase at the same rate. Government legislation and specific energy conservation targets have been set for overall energy reduction on a national basis by many countries. Additionally, government incentives are available to reduce energy usage and environmental impact. Gough [2] points out that the recent nuclear disaster in Japan has prompted the Japanese government to take a more active role in its serious drive to reduce energy use. Recently, additional countries have also started to adopt that approach, making the development of enhanced heat transfer even more important. Reay [1] notes 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics