Falling-film evaporation over horizontal rectangular tubes: Part I—Experimental resultsÉvaporation en film tombant sur des tubes rectangulaires horizontaux: Partie I - Résultats expérimentaux

https://doi.org/10.1016/j.ijrefrig.2020.07.018Get rights and content

Highlights

  • Experimentally measured falling-film evaporation heat transfer coefficients.

  • Effects of temperature, flow rate, tube spacing, tube row, and heat flux discussed.

  • Newly observed heat transfer coefficient trend that is not seen in round tubes.

  • New trend due to film breakdown quantified and supported by force balance.

Abstract

An experimental study on falling-film evaporation on horizontal rectangular tubes, the external geometry of extruded flat, microchannel tubes, was conducted. Previous falling-film evaporation studies have focused on horizontal and vertical round tubes, and the usage of microchannel tubes provides a promising new heat exchanger design option. The test sections used in this study have external dimensions of 203 × 1.42 × 27.4 mm (length × width × height) and were tested with water as the refrigerant. Heat transfer coefficients were measured for saturation temperatures from 10 to 30°C, test section spacings from 5 to 15 mm, heat fluxes from 10 to 20 kW m−2, and film Reynolds numbers of 48 to 544. This paper, Part I, reports the experimental methodology and the results, while in the companion paper, Part II, models for predicting heat transfer are presented.

Introduction

Falling-film evaporators have applications in refrigeration, desalination, and other areas. They have several advantages over flooded evaporators: lower refrigerant charge, minimal pressure drop, and operation over small temperature differences. Several possible configurations for falling-film evaporators, based primarily on films falling over horizontal or vertical round tubes, have been investigated. One promising heat exchanger configuration that has received less attention for external falling films is the use of flat microchannel tubes. Microchannel tubes have a thin, rectangular profile with a series of small internal ports, each with a hydraulic diameter < ~1 mm. In an evaporator utilizing microchannel tubes, the tubes could be orientated horizontally in a vertical array with internal single- or two-phase flow and an external evaporating thin film. Microchannel tubes possess several characteristics that make them ideal for such a configuration: high surface area-to-volume ratios for the internal and external flows, the ability to withstand high internal pressures, and a low refrigerant charge. This two-part study focuses on the heat transfer of falling film evaporation on horizontal rectangular tubes, representative of the external profile of microchannel tubes. Experiments and results are discussed here in Part I, while models based on these results and physical insights are presented in the companion paper, Part II.

Reviews of the related literature have been compiled by Thome (1999; 2009), Ribatski and Jacobi (2005), Mitrovic (2005), and Fernández-Seara and Pardiñas (2013). Thome (1999) provided an overview of significant articles prior to 1994 and a more detailed discussion of studies published from 1994 to 1999. A later publication (Thome, 2009) expanded on this review with additional discussion on the advantages and disadvantages of horizontal tube falling-film evaporators, thermal design considerations, falling-film modes, and recent work. Ribatski and Jacobi (2005) critically examined the literature on flow pattern studies, plain tubes, enhanced surfaces, tube bundles, and mathematical and empirical models for heat transfer, with a focus on studies related to refrigeration applications. Mitrovic (2005) provided a review focusing on flow pattern correlations, and included a discussion of the Reynolds numbers and other dimensionless numbers used by different authors. Fernández-Seara and Pardiñas (2013) reviewed the falling-film literature related to refrigerants, and compared falling-film evaporation with pool boiling. Studies of falling-film evaporation on horizontal round tubes have explored the influence of a wide range of parameters in both the convective evaporation and boiling regimes. Table 1 summarizes the findings of these studies, discussing the influence of flow rate, dryout, heat flux, tube spacing, angle around tube, tube diameter, and vapor flow.

Several studies have investigated falling films over horizontal round tubes, with heat transfer studies of evaporative and boiling conditions focusing on a range of fluids, flow rates, surface conditions, and other factors. However, investigations of films falling over rectangular tubes have been limited to studies of adiabatic flow transitions, single-phase heat transfer, and flow mechanisms. The present study investigates falling-film evaporation heat transfer, measuring heat transfer coefficients for a range of mass flow rates, heat fluxes, temperatures, and tube spacings. The results of the experimental study are presented in Part I, while methods and models for predicting heat transfer are presented in Part II.

Section snippets

Experimental approach

The test facility enables simultaneous heat transfer measurements and flow visualization of evaporating films falling over flat horizontal tubes at sub-atmospheric pressures. It was previously used to conduct a flow visualization study quantifying droplet and wave characteristics (Bustamante and Garimella, 2014). In the present study, heat transfer tests were conducted with water at saturation temperatures of 10 to 30°C. A schematic and photograph of this test facility are seen in Fig. 1. It

Data analysis

Before taking each heat transfer data point, the flow regime was recorded. Flow transition and heat transfer results are from data collected from the first test section (Test Section 1). The other two test sections were used to determine dominant flow mechanisms in falling film evaporation over horizontal tube banks, discussed in Bustamante and Garimella (2014). This was done because of substantial film breakdown on Test Sections 2 and 3 (see Section 5.4). Steps such as modified geometry,

Results

The observed flow regimes are shown in Fig. 4, where the film Reynolds number is determined from the flow rate at the top of Test Section 1. In the test matrix considered in the present study, there were a total of 57 data points in the droplet mode, 112 data points in the droplet-jet mode, 21 data points in the jet mode, and 134 data points in the jet-sheet mode. These data are plotted against test section spacing, temperature, heat flux, and flow rate in Fig. 5. Here, the spacing is the gap

Influence of temperature

Fig. 6 plots the heat transfer coefficient versus mass flow rate, and Nusselt number versus film Reynolds number for the four temperatures considered in this study. The heat transfer coefficient increases as temperature increases from 10 to 20°C, but then decreases when temperature increases further from 20 to 30°C. The spread in the data at each mass flow rate is due to the different heat flux and tube spacing of each test condition. To determine the cause of these heat transfer trends with

Conclusions

Falling-film evaporation experiments on horizontal rectangular tubes were conducted to measure heat transfer coefficients. Test sections with external dimensions of 203 × 1.42 × 27.4 mm (length × width × height) were used with water as the refrigerant. Heat transfer coefficient measurements were taken for saturation temperatures from 10 to 30°C, test section spacings from 5 to 15 mm, heat fluxes from 10 to 20 kW m−2, and film Reynolds numbers of 48 to 544. These experiments found that the heat

Declaration of Competing Interest

None.

Acknowledgements

The authors gratefully acknowledge the support provided by the U.S. Office of Naval Research under contract number N000140710847 for this research.

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