Experimental and Visual Study on Flow Patterns and Pressure Drops in U-Tubes

In single- and two-phase flow heat exchangers (in particular "coils"), besides the straight tubes there are also many singularities, in particular the 180° return bends (also called return bends or U-bends). However, contrary to the literature concerning pressure drops and heat transfer in straight tubes, where many experimental data and predicting methods are available, only a limited number of studies concerning U-bends can be found. Neither reliable experimental data nor proven prediction methods are available. Indeed, flow structure, pressure drop and heat transfer in U-bends are an old unresolved design problem in the heat transfer industry. Thus, the present study aims at providing further insight on two-phase pressure drops and flows patterns in U-bends. Based on a new type of U-bend test section, an extensive experimental study was conducted. The experimental campaign covered five test sections with three internal diameters (7.8, 10.8 and 13.4mm), five bend diameters (24.8, 31.7, 38.1, 54.8 and 66.1mm), tested for three orientations (horizontal, vertical upflow and vertical downflow), two fluids (R134a and R410A), two saturation temperatures (5 and 10 °C) and mass velocities ranging from 150 to 1000kg s-1m-2. The flow pattern observations identified were stratified-wavy, slug-stratified-wavy, intermittent, annular, dryout and mist flows. The effects of the U-bend on the flow patterns were also observed. A total of 5655 pressure drop data were measured at seven different locations in the test section ( straight tubes and U-bend) providing a total of almost 40'000 data points. The straight tube data were first used to improve the actual two-phase straight tube model of Moreno-Quibén and Thome. This updated model was then used to developed a two-phase U-bend pressure drop model. Based on a comparison between experimental and predicted values, it is concluded that the new two-phase frictional pressure drop model for U-bends successfully predicts the new experimental data. The present work is a step forward on the LTCMs flow pattern based work on two-phase flow and heat transfer inside horizontal round tubes: (i) generalized flow pattern map, (ii) flow boiling heat transfer model, (iii) convective condensation model, (iv) two-phase frictional pressure drop model for straight tubes, and (v) two-phase frictional pressure drop model for singularities - U-bends.

Boiling on a tube bundle

Francesco Agostini

The complexity of the two-phase flow in a tube bundle presents important problems in the design and understanding of the physical phenomena taking place. The working conditions of an evaporator depend largely on the dynamics of the two-phase flow that in turn influence the heat exchange and the pressure drop of the system. A characterization of the flow dynamics, and possibly the identification of the flow pattern in the tube bundle, in thus expected to lead to a better understanding of the phenomena and to reveal on the mechanisms governing the tube bundle. Therefore, the present study aims at providing further insights into two-phase bundle flow through a new visualization system able to provide for the first time a view of the flow in the core of a tube bundle. In addition, the measurement of the light attenuation of a laser beam through the two-phase flow and measurement of the high frequency pressure fluctuations with a piezo-electric pressure transducer are used to characterize the flow. The design and the validation of this new instrumentation also provided a method for the detection of dry-out in tube bundles. This was achieved by a laser attenuation technique, flow visualization, and estimation of the power spectrum of the pressure fluctuation. The current investigation includes results for two different refrigerants, R134a and R236fa, three saturations temperatures Tsat = 5, 10 and 15 °C, mass velocities ranging from 4 to 40 kg/sm2 in adiabatic and diabatic conditions (several heat fluxes). Measurement of the local heat transfer coefficient and two-phase frictional pressure drop were obtained and utilized to improve the current prediction methods. The heat transfer and pressure drop data were supported by extensive characterization of the two-phase flow, which was to improve the understanding of the two-phase flow occurring in tube bundles.

EPFL2008

Heat Transfer and Pressure Drop in the Dryout Region of Intube Evaporation with Refrigerant/Lubricant Mixtures.

Daniel Favrat, John Richard Thome, Olivier Zürcher

A summary of the objectives and conclusions reached in this project are: •A litterature search on post dryout heat transfer for pure refrigerants and for refrigerant-oil mixtures was performed and a comprehensive review written (Chapters 9, 10, and 11); •The correct thermodynamic definition of the heat transfer coefficient for evaporation of refrigerant-oil mixtures was presented and then used in the project, i.e. defining the heat transfer coefficient using the local bubble point temperature Tbub of the refrigerant-oil mixture rather than with the saturation temperature of the pure refrigerant Tsat incorrectly used in prior studies (Chapter 3); •A new generalized approach, that is simple to implement, was developed for predicting bubble point temperatures of refrigerant-oil mixtures for miscible oils that can be applied to any refrigerant-oil combination and is accurate over the local oil concentration range from 0-50 wt.% oil range confronted in direct- expansion evaporators with 0-5 wt.% oil in their refrigerant charge (Chapter 4); •A new thermodynamic method for preparation of temperature-enthalpy- vapor quality (T-h-x) curves for refrigerant-oil mixtures for general application was developed and validated against test data (Chapters 5 and 6), including a simple expression for calculating local oil concentrations as a function of vapor quality; •New, accurate thermodynamic methods for preparation of temperature-enthalpy-vapor (T-h-x) curves for the refrigerant blend R-407C and R-407C/oil mixtures were developed for thermal design of evaporators and for the present experimental test, and was validated against measured test data (Chapter 12); •Recommendations were made on how to include oil effects and refrigerant-oil T-h-x curves into evaporator design methods (Chapter 7); •The effects of oil on set-point parameters for control devices were analyzed and some recommendations made (Chapter 8); •An online oil concentration measurement system was perfected using a high accuracy density flowmeter that allowed oil concentrations to be measured accurately and continuously during experiments, such that oil holdup in the heat transfer test sections at high vapor quality could be identified and the correct inlet oil concentrations be measured, and thus utilized to reduce experimental data; a simpler industrial method for applying the measurement system was also developed and presented (Chapter 15); •Experimental results (local heat transfer coefficients, two-phase pressure drops and some two-phase flow patterns) were obtained for R-134a and R-134a/oil mixtures evaporating in plain and microfinned tubes over a wide range of test conditions (Chapters 16 and 17); •Experimental results (local heat transfer coefficients, two-phase pressure drops and some two-phase flow patterns ) were obtained for R-407C and R- 407C/oil mixtures evaporating in plain and microfinned tubes over a wide range of test conditions (Chapters 18 and 19); •The recently proposed flow boiling model and flow pattern map of Kattan-Thome-Favrat, that importantly models heat transfer coefficients based on local flow pattern and predicts the onset of dryout in horizontal tubes and local heat transfer coefficients in partial dryout regimes, was described in detail; the flow pattern map is also useful to designers wishing to design in specific flow pattern regimes and avoid inefficient heat transfer regimes, such as mist flow and stratified flow (Chapters 20 and 21); •The stratified-wavy flow regime model was modified based on the new test data at high vapor quality for pure R-134a and R-407C, which increased the accuracy of the method significantly in this flow regime, now referred to as the “modified” Kattan- Thome-Favrat flow boiling model (Chapter 22); •The modified Kattan- Thome-Favrat flow boiling model and flow pattern map were shown to accurately predict R-407C heat transfer and flow pattern data and hence is recommended for industrial use for designing direct-expansion evaporators with refrigerant blends [the original version was also verified for R-402 and R-404A blends, see Chapter 21] (Chapter 22); •The effect of local refrigerant-oil viscosity on liquid-phase convection in flow boiling heat transfer and on flow patterns was successfully incorporated into the modified Kattan-Thome-Favrat model, establishing the first flow boiling design method that predicts oil effects on heat transfer (with quite reasonable accuracy too), and included a simple method for calculating the values of µref-oil for use in the model (Chapter 23); •The oil effects on microfin heat transfer coefficients were analyzed and heat transfer augmentation ratios were determined for R-134a/oil and R-407C/oil mixtures (Chapter 25); •Plain tube, two-phase pressure drop gradients for R-134a and R-407C at three mass velocities were compared to the Friedel frictional two-phase pressure drop correlation, attaining accurate results and also microfin pressure drop augmentation ratios were determined (Chapter 25); •The ratio of [µoil/µref]mw, derived from the Arrhenius logrithmic viscosity mixing rule and local oil concentration w, was shown to accurately predict the effect of oil on two-phase pressure drop gradients at high vapor qualities, with an empirical exponent m = 0.18355 found for R-134a/oil mixtures (no foaming) and an empirical expression for m for R-407C/oil mixtures (with foaming). This ratio incorporated into the Friedel correlation accurately predicts two-phase pressure drops of evaporating refrigerant-oil flows (Chapter 25).

1996

Macro-to-Microchannel Transition in Two-Phase Flow and Evaporation

Chin Lee Ong

Continuous development of highly efficient and compact cooling elements are driven by extensive developments of Micro-Electro-Mechanical Systems (MEMS) devices such as microprocessors or micro high-powered laser systems. As microprocessors continue to head in the direction of miniaturization, it was quickly realized that the major obstacle to overcome was "heat". This justified the need for the development of a new generation of liquid cooling technologies that involves phase-change processes. However, significant differences in two-phase transport phenomena have been reported in the microscale sized channels as compared to conventional macroscale channels. The classification of macroscale, mesoscale and microscale channels with respect to two-phase processes is still an open question. This research project focuses on investigating the macro-to-microscale transition during flow boiling in small scale channels of various size and refrigerants to investigate the effects of channel confinement on flow boiling heat transfer, two-phase flow patterns, pressure drop, critical heat flux and liquid film stratification in a single circular horizontal channel. In this project, we systematically investigated the trends observed in the heat transfer data, pressure drop, critical heat flux and film stratifications for wide range of conditions as a function of flow pattern, creating a uniquely comprehensive experimental database for R134a (high pressure), R236fa (medium pressure) and R245fa (low pressure) in the 1.03, 2.20 and 3.04 mm channels. From a heat transfer viewpoint, the prediction of the heat transfer requires first the prediction of the two-phase flow regimes in these small scale channels. So, in that respect an improved flow pattern map has been proposed and by determining the flow patterns transitions existing under different conditions, the effects of the flow regimes and channel dimensions on flow boiling heat transfer is thus explained. A flow pattern based heat transfer prediction model, i.e. the three-zone model showed good agreement in the prediction of heat transfer coefficients corresponding to the isolated bubble (IB) and coalescing bubble (CB) flows. As for the two-phase pressure drops, various statistical comparisons have been made by comparing the pressure drop database with various macroscale and microscale prediction methods, finding that three macroscale and two microscale methods predicted the pressure drop database with good accuracy. From this research, a distinctive peak in the CHF has been identified when comparing the experimental CHF database for 0.50, 0.80, 1.03, 2.20 and 3.04 mm channels (the first two from previous study). A new CHF prediction method was developed that is applicable for the CHF prediction of single, square multi-microchannels and split flow multi-channels. Finally, the processing of the high speed flow visualization images provided the interpretation of the lower threshold of the macro-microscale transition. As an approximate rule, the lower threshold of macro-scale flow is Co= 0.3-0.4 while the upper threshold of the symmetric microscale flow is Co≈1.