Simulation and experimental validation of pulsating heat pipes

The one-dimensional transient numerical code of JJ Cooling Innovation has been significantly improved to simulate the thermal/hydraulic performance of pulsating heat pipes (PHPs). Using a mechanistic approach, the liquid slug and vapor plug flows and their interaction with the surrounding wall are modeled. The heat transfer is predicted using the Three-Zone Model of flow boiling in microchannels, which includes liquid film evaporation and liquid film condensation of vapor plugs and convection of liquid slugs. Discretizing the entire PHP serpentine into one continuous grid, the code solves all variables based on local conditions. The code allows the user to define the working fluid, the channel's shape, material and wall roughness, the serpentine's pitch, the geometrical parameters of evaporator, adiabatic region and condenser zones, and the operating parameters. The boundary conditions in the evaporator zone can either be constant heat flux or constant temperature, while on the condenser side, the coolant temperature is constant as well as the heat transfer coefficient. The code works without any fitting factor and predicts all local variables in the 1D grid as a function of time, including the liquid film thickness and the onset of boiling for the formation of new vapor plugs within the evaporating and adiabatic zones, if local nucleation conditions are reached. Growth, collapse and coalescence of generated bubbles are taken into account. The film thermal resistance is corrected to a radial conduction model. The implementation of the initial liquid film thickness laid behind a liquid slug is updated to include the effect of the bubble velocity and acceleration as well as the length of the leading slug. The nucleation of new vapor plug has also been updated. The onset of nucleation is only dependent on the local thermodynamic state. If the threshold is reached, a new method is used to simulate micro-bubbles formation and coalescence to a channel-size vapor plug. These modifications greatly improve the code accuracy and stability allowing simulations with high pressure fluid rather than ethanol only. Furthermore, in the on-going EU project Pulsating Heat Pipes for Hybrid Propulsion Systems (PHP2), a new PHP test bench has been designed and built at Provides Metalmeccanica. A copper tube PHP with a 19-turn of 2.0 mm internal diameter and 12 mm tube pitch was tested over a wide range of imposed heat loads. A microchannel water-cooling plate was used at the condenser side with an inlet temperature of 20 degrees C. Then, tests were done for vertical and horizontal orientations with R1233zd(E) as the working fluid. The code, which was previously validated for ethanol PHP test data from KAIST, is now successfully validated for the present experimental data points. Accurate results were obtained from low heat loads up to 250 W for a set of 49 data points, predicting 97% within an error bandwidth of 30%.

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

Numerical Modeling of Annular Laminar Film Condensation in Circular and Non-Circular Micro-Channels under Normal and Micro-Gravity

Stefano Nebuloni

A theoretical and numerical model to predict film condensation heat transfer in mini, micro and ultra micro-channels of different internal shapes is presented in this thesis. The model is based on a finite volume formulation of the Navier-Stokes and energy equations and it includes the contributions of the unsteady terms, surface tension, axial shear stresses, gravitational forces and wall thermal conduction. Notably, interphase mass transfer and near-to-wall effects (disjoining pressure) are also included. This model has been validated versus various benchmark cases and versus published experimental results from three different laboratories, predicting micro-channel heat transfer data with an average error of 20 % or better. The conjugate heat transfer problem arising from the coupling between the thin film fluid dynamics, the heat transfer in the condensing fluid and the heat conduction in the channel wall has been studied and analyzed. The work has focused on the effects of three external wall boundary conditions: a uniform wall temperature, a non uniform wall heat flux and single-phase convective cooling. The thermal axial and peripheral conduction occurring in the wall of the channel can affect the behavior of the condensate film, not only because it redistributes the heat, but also because the annular laminar film condensation process is dependent on the local saturation to wall temperature difference. When moving from mini to micro and ultra-micro channels, the results shows that the axial conduction effects can become very important in the prediction of the wall temperature profile and they can not be ignored. Under these conditions, the overall performances of the heat exchanger become dependent not only on the fluid properties and the operative conditions but also on the geometry and wall material. Results obtained for steady state conditions are presented for circular, elliptical and flattened shape cross sections for R-134a and ammonia, for hydraulic diameters between 10 µm and 3 mm. Microscale condensation finds applications in heat pipes and compact heat exchangers for electronic equipment or spacecraft thermal control, in automotive condensers, in residential air conditioning and in refrigeration applications: the influence of the steady or unsteady gravitational field and of the inertia forces on the flow field and consequently on the heat transfer performances is investigated allowing the model to be applied as a design and optimization tool for enhanced heat exchangers.

EPFL2010

Evaporation du réfrigérant HFC 407C sur des tubes lisses ou à surface améliorée.

Daniel Favrat, John Richard Thome, Olivier Zürcher

The substitution of CFC refrigerants in refrigeration systems, heat pumps and organic Rankine cycles for heat recovery, requires good methods for predicting heat transfer of substitute fluids. The measurements in the LENI test facility (concentric tubes with water flowing in a countercurrent flow) with HFC 407C, HFC 134a, HCFC 123, HFC 404A and HFC/HCFC 402A provide a new data band for new refrigerants, and allow a coherent comparison with old refrigerants CFC 11, CFC 12, CFC/HCFC 502 and with existing correlations. The existing correlations were found to be inadequate. Because of this work, an improved flow pattern map and flow boiling model were developed, which resulted in a substantial progress in the accurate predict of heat transfer in plain, horizontal tubes for refigerants without oil. The Kattan et al. (1996, [6]) correlation was programmed to calculate and compare predicted heat transfer coefficients to the new HFC 407C test data. The flow pattern map proposed by Kattan et al. (1996) was also programmed and compared to flow regimes observed for HFC 407C. It predicted the HFC 407C flow pattern data quite accurately. The original objective of the HFC 407C measurements was the validation of the Kattan et al. (1996) correlation applied to a zeotropic refrigerant blend. Local flow boiling heat transfer coefficients were measured for HFC 407C evaporating inside a microfin and plain tube. In addition, microfin heat transfer augmentation relative to a plain tube was investigated. The presence of oil in the evaporator had a complex effect on heat transfer coefficients. Local flow boiling heat transfer coefficients were measured for refrigerant HFC 407C ester oil mixtures (Mobil EAL Arctic 68). A new thermodynamic approach for modeling mixtures of zeotropic refrigerant blends and lubricating oils was also developed. A very high accuracy, straight vibrating tube type of density flowmeter was used to measure oil concentrations of flowing HFC 407C oil mixtures. The test covered oil concentration from 0.5-5 wt.% oil. A method was proposed to predict thermodynamic and transport properties of the refrigerant-oil mixtures. In addition, a first empirical approach was proposed for predicting heat transfer for boiling of refrigerant-oil mixtures. Based on this work, the following phenomena important to the problem were identified that require further investigation: • oil retention in the evaporator tubes that was found and clearly proved in the present project. • the formation of foam in the HFC 407C/oil mixture (but not in the previous HFC 134a/oil tests) •the effect of oil on flow patterns, in particular stratified and stratified-wavy flows.

1996

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

Daniel Favrat, John Richard Thome, Olivier Zürcher

1996

Numerical Modeling of Annular Laminar Film Condensation in Circular and Non-Circular Micro-Channels under Normal and Micro-Gravity

Stefano Nebuloni

EPFL2010

Evaporation du réfrigérant HFC 407C sur des tubes lisses ou à surface améliorée.

Daniel Favrat, John Richard Thome, Olivier Zürcher

1996