Falling film evaporation on enhanced tubes, part 1: Experimental results for pool boiling, onset-of-dryout and falling film evaporation

Horizontal falling film evaporators have the potential of displacing flooded evaporators as the standard in the refrigeration industry due to compelling advantages, such as smaller refrigerant charge and lower pressure drop. However, there is a need to improve the understanding of falling film evaporation mechanisms to provide accurate thermal design methods. In this work, falling film evaporation measurements on a single tube, a vertical row of ten horizontal tubes and a small tube bundle with three rows of 10 tubes each are presented. Two enhanced boiling tubes, the Wolverine Turbo-B5 and the Wieland Gewa-B5, were tested using R-134a and R-236fa. The tests were carried out at a saturation temperature of T-sat = 5 degrees C, film Reynolds numbers ranging from 0 to 3000, and heat fluxes between 15 and 90 kW m(-2) in pool boiling and falling film configurations. A visualization study was performed under diabatic conditions to study the flow. (C) 2011 Elsevier Ltd and IIR. All rights reserved.

Heat Transfer and Visualization of Falling Film Evaporation on a Tube Bundle

Marcel Christians

Horizontal falling film evaporators have the potential of displacing flooded evaporators in industry, due to advantages such as lower required refrigerant charge and lower pressure drop. However, there is a need to improve the understanding of falling film evaporation mechanisms to provide accurate thermal design methods. In this work, the existing LTCM falling film facility was utilized to perform falling film evaporation measurements on a single tube, a vertical row of horizontal tubes and a small tube bundle. Two enhanced boiling tubes, namely the Wolverine Turbo-B5 and the Wieland Gewa-B5, were tested using R-134a and R-236fa. The tests were carried out at a constant saturation temperature of Tsat = 5°C, liquid film Reynolds numbers ranging from 0 to 3000, and heat fluxes between 15 and 90 kW/m2 in pool boiling and falling film configurations. A visualization study was performed under adiabatic and diabatic conditions (in both single-array and bundle configurations) to study the flow. The physical phenomena governing the falling film evaporation process have been studied, and insight into their effects on the performance of tube bundles has been gained. Measurements of the local heat transfer coefficient were obtained and utilized to generate new prediction methods, including a method for predicting the onset-of-dryout film flow rate during falling film evaporation, local pool boiling and falling film heat transfer prediction methods and a falling film multiplier prediction method.

EPFL2010

Falling film evaporation on a single tube and on a tube bundle

Jean-François Roques

This study is a contribution for the improvement of horizontal falling film heat exchangers. This type of evaporator has the potential to be widely used in the petrochemical industry, for the sea water desalination, or in the large refrigeration systems. An experimental test facility has been constructed in order to study the evaporation of the liquid refrigerant R-134a flowing on a vertical array of horizontal copper tubes with a length of 0.55 meter and a diameter of 19.1mm. Special care has been taken to achieve uniform liquid distribution along the top tube. Four types of tubes were tested including a plain tube, two tubes with mechanically enhanced boiling surfaces (Turbo-BII HP and Gewa-B) and a porous coated tube (High-Flux). Tube arrays with three different tube pitches were tested. Measurements were performed at three different nominal heat flux levels over a wide range of liquid overfeed. For comparison, heat transfer coefficients in pool boiling conditions were measured. The experimental parameters are the following: the liquid film Reynolds numbers from 0 to 3000, heat fluxes between 20 and 60kW/m2 and tube pitches from 22.3 and 25.5mm. The heat transfer coefficients have been measured locally at the mid point of each of the ten tubes. The study shows that there are two types of fundamental behavior characterized by a particular film Reynolds number: Over a threshold value of this Reynolds number, the nucleate boiling in the film governs the heat transfer and this heat transfer coefficient is independent of the Reynolds number. Below this threshold value, the heat transfer coefficients drop drastically due to the formation of local dry patches. The heat transfer coefficient during falling film evaporation was found to be approximately 1.3 times larger than the heat transfer coefficient during pool boiling and this multiplier was found to be dependant on the heat flux and tube spacing but not the flow mode. Finally, the understanding of the physical phenomena governing the falling film evaporation of liquid refrigerants has been improved. Furthermore, a method for predicting the film Reynolds number at the onset of dry patch formation has been developed and a heat transfer correlation for boiling iii falling films proposed. These represent significant improvements for the design of falling film evaporators.

EPFL2004

Boiling on a Tube Bundle

Eugene van Rooyen

The complexity of two-phase flow boiling on a tube bundle presents many challenges to the understanding of the physical phenomena taking place. It is important to quantify these numerous heat flow mechanisms in order to better describe the performance of tube bundles as a function of the operational conditions. In the present study, the bundle boiling facility at the Laboratory of Heat and Mass Transfer (LTCM) was modified to obtain high-speed videos to characterise the two-phase regimes and some bubble dynamics of the boiling process. It was then used to measure heat transfer on single tubes and in bundle boiling conditions. Pressure drop measurements were also made during adiabatic and diabatic bundle conditions. New enhanced boiling tubes from Wolverine Tube Inc. (Turbo-B5) and the Wieland-Werke AG (Gewa-B5) were investigated using R134a and R236fa as test fluids. The tests were carried out at saturation temperatures Tsat of 5°C and 15°C, mass flow rates from 4 to 35 kg/m2s and heat fluxes from 15 to 70 kW/m2, typical of actual operating conditions. The flow pattern investigation was conducted using visual observations from a borescope inserted in the middle of the bundle. Measurements of the light attenuation of a laser beam through the intertube two-phase flow and local pressure fluctuations with piezo-electric pressure transducers were also taken to further help in characterising the complex flow. Pressure drop measurements and data reduction procedures were revised and used to develop new, improved frictional pressure drop prediction methods for adiabatic and diabatic two-phase conditions. The physical phenomena governing the enhanced tube evaporation process and their effects on the performance of tube bundles were investigated and insight gained. A new method based on a theoretical analysis of thin film evaporation was used to propose a new correlating parameter. A large new database of local heat transfer coefficients were obtained and then utilised to generate improved prediction methods for pool boiling and bundle boiling, including a method for predicting the onset of dryout.