Analysis of flow maldistribution in fin-and-tube evaporators for residential air-conditioning systems

This thesis is concerned with the effects of flow maldistribution in fin-and-tube A-coil evaporators for residential air-conditioning and compensation potentials with regards to system performance. The goal is to create a better understanding of flow maldistribution and the involved physical phenomenons. Moreover, the study investigates the individual and combined effects of non-uniform inlet liquid/vapor distribution, different feeder tube bending and non-uniform airflow. In addition, the possible compensation of these maldistribution sources is investigated by control of individual channel superheat by distributing individual channel mass flow rate continuously (perfect control). The compensation method is compared to the use of a larger evaporator in order to study their trade-off in augmenting system performance (cooling capacity and COP). The studies are performed by numerical modeling in the object-oriented programming language Modelicar and by using the commercial modeling environment Dymola 7.4 (2010). The evaporator model needs to be capable of predicting the flow distribution and circuitry effects, and for these reasons the dynamic distributed one-dimensional mixture two-phase flow model is implemented. The model is verified in steady state with commercial software Coil-Designer (Jiang et al., 2006) and compared to steady state experiments with acceptable results considering the unknown degrees of flow maldistribution for these experiments. Furthermore, the system dynamics in the model were validated and showed that a slip flow model need be used. A test case 8.8 kW residential air-conditioning system with R410A as refrigerant is chosen as baseline for the numerical investigations, and the simulations are performed at standard rating conditions from ANSI/AHRI Standard 210/240 (2008). The investigations are performed on a simplified evaporator tube circuitry (two straight channels), a face split evaporator circuitry and an interlaced evaporator circuitry. The first case is a generic study and serves to provide general results independent of specific type of tube circuitry. The second and third cases are standard tube circuitry designs and these results

[1]  S. M. Ghiaasiaan.  TWO-PHASE FLOW, BOILING AND CONDENSATION IN CONVENTIONAL AND MINIATURE SYSTEMS , 2007 .

[2]  Hubertus Tummescheit,et al.  Design and Implementation of Object-Oriented Model Libraries using Modelica , 2002 .

[3]  Michael Tiller,et al.  Introduction to Physical Modeling with Modelica , 2001 .

[4]  David A. Yashar,et al.  Air Flow Distribution through an A-Shaped Evaporator under Dry and Wet Coil Conditions , 2010 .

[5]  L. Larsen,et al.  Compensation of flow maldistribution in fin-and-tube evaporators for residential air-conditioning , 2011 .

[6]  Chun-Lu Zhang,et al.  A generalized moving-boundary model for transient simulation of dry-expansion evaporators under larger disturbances , 2006 .

[7]  James E. Braun,et al.  Evaluation of a hybrid method for refrigerant flow balancing in multi-circuit evaporators , 2009 .

[8]  M. Shah.  A general correlation for heat transfer during film condensation inside pipes , 1979 .

[9]  H. Müller-Steinhagen,et al.  A simple friction pressure drop correlation for two-phase flow in pipes , 1986 .

[10]  S. Zivi.  Estimation of Steady-State Steam Void-Fraction by Means of the Principle of Minimum Entropy Production , 1964 .

[11]  Jonas Eborn,et al.  AirConditioning - a Modelica Library for Dynamic Simulation of AC Systems , 2005 .

[12]  A. F. Mills.  Heat Transfer (2nd edition) , 1999 .

[13]  Morten Willatzen,et al.  A general dynamic simulation model for evaporators and condensers in refrigeration. Part I: moving-boundary formulation of two-phase flows with heat exchange , 1998 .

[14]  Christoph Richter,et al.  Proposal of New Object-Oriented Equation-Based Model Libraries for Thermodynamic Systems , 2008 .

[15]  Guoliang Ding,et al.  Experimental validation of void fraction models for R410A air conditioners , 2009 .

[16]  J. Thome,et al.  Investigation of Flow Boiling in Horizontal Tubes: Part II, Development of a New Heat Transfer Model for Stratified-Wavy, Dryout and Mist Flow Regimes , 2005 .

[17]  J. Thome.  Boiling of new refrigerants: a state-of-the-art review , 1996 .

[18]  Haobo Jiang,et al.  Development of a Simulation and Optimization Tool for Heat Exchanger Design , 2003 .

[19]  Young-Chul Kwon,et al.  An improved method for analyzing a fin and tube evaporator containing a zeotropic mixture refrigerant with air mal-distribution , 2003 .

[20]  J. M. Jensen,et al.  Dynamic modelling of thermo-fluid systems - with focus on evaporators for refrigeration , 2003 .

[21]  Brian Elmegaard,et al.  Experimental comparison of the dynamic evaortor response using homogeneous and slip flow modeling , 2011 .

[22]  V. Gnielinski.  New equations for heat and mass transfer in turbulent pipe and channel flow , 1976 .

[23]  L. Larsen,et al.  Low charge system behaviour: interaction of heat exchanger volumes and charge. , 2010 .

[24]  Olaf Bauer,et al.  Modelling of Two-Phase Flows with Modelica , 1999 .

[25]  J. Thome,et al.  Convective boiling and condensation - Third edition , 1994 .

[26]  Piotr A. Domanski,et al.  Effects of Non-Uniform Refrigerant and Air Flow Distributions on Finned- Tube Evaporator Performance , 2003 .

[27]  Reinhard Radermacher,et al.  A-Type Heat Exchanger Simulation Using 2-D CFD for Airside Heat Transfer and Pressure Drop , 2008 .

[28]  B. Elmegaard,et al.  Analysis of refrigerant mal-distribution: in fin-and-tube evaporators , 2009 .

[29]  A. M. Judd.  Convective Boiling and Condensation. , 1973 .

[30]  David A. Yashar,et al.  Application of an Evolution Program for Refrigerant Circuitry Optimization | NIST , 2007 .

[31]  B. Elmegaard,et al.  Modelling distribution of evaporating CO2 in parallel minichannels , 2010 .

[32]  L. Larsen,et al.  Effect of refrigerant mal-distribution in fin-and-tube evaporators on system performance , 2009 .

[33]  P. Lettieri,et al.  An introduction to heat transfer , 2007 .

[34]  Guoliang Ding,et al.  On three forms of momentum equation in transient modeling of residential refrigeration systems , 2009 .

[35]  J. Thome,et al.  Flow pattern based two-phase frictional pressure drop model for horizontal tubes, Part II: New phenomenological model , 2007 .

[36]  Arne Jakobsen,et al.  Experimental evaluation of the use of homogeneous and slip-flow two-phase dynamic models in evaporator modelling , 1999 .

[37]  K. Gungor,et al.  A general correlation for flow boiling in tubes and annuli , 1986 .

[38]  A. Ghajar,et al.  Comparison of void fraction correlations for different flow patterns in horizontal and upward inclined pipes , 2007 .

[39]  Lothar Dr. Siegert,et al.  A refrigeration system , 2008 .

[40]  Peter A. Fritzson,et al.  Principles of object-oriented modeling and simulation with Modelica 2.1 , 2004 .

[41]  Rüdiger Franke,et al.  Stream Connectors - An Extension of Modelica for Device-Oriented Modeling of Convective Transport Phenomena , 2009 .

[42]  Vikrant Aute,et al.  CoilDesigner: a general-purpose simulation and design tool for air-to-refrigerant heat exchangers , 2006 .

[43]  Piotr A. Domanski,et al.  Potential benefits of smart refrigerant distributors. Final report. , 2003 .

[44]  J. Thome,et al.  Investigation of Flow Boiling in Horizontal Tubes: Part I, A New Diabatic Two-Phase Flow Pattern Map , 2005 .

[45]  L. Larsen,et al.  Performance of residential air-conditioning systems with flow maldistribution in fin-and-tube evaporators , 2011 .

[46]  Chi-Chuan Wang,et al.  Heat transfer and friction correlation for compact louvered fin-and-tube heat exchangers , 1999 .

[47]  James E. Braun,et al.  Application of CFD Models to Two-Phase Flow in Refrigerant Distributors , 2005 .

[48]  Brian Elmegaard,et al.  Modelling refrigerant distribution in microchannel evaporators , 2009 .

[49]  M. Shah.  Chart correlation for saturated boiling heat transfer: Equations and further study , 1982 .

[50]  K. Gungor,et al.  Simplified general correlation for saturated flow boiling and comparisons of correlations with data , 1987 .

[51]  David A. Yashar,et al.  Measurement of Air-Velocity Profiles for Finned-Tube Heat Exchangers Using Particle Image Velocimetry , 2008 .

[52]  Hilding Elmqvist,et al.  Modelica — A unified object-oriented language for physical systems modeling , 1997 .

[53]  S. L. Smith.  Void Fractions in Two-Phase Flow: A Correlation Based upon an Equal Velocity Head Model , 1969 .

[54]  H. Itō.  Pressure Losses in Smooth Pipe Bends , 1960 .

[55]  J. M. Coulson,et al.  Heat Transfer , 2018, Finite Element Method for Solids and Structures.

[56]  John R. Thome,et al.  Measurement of dynamic void fractions in stratified types of flow , 2005 .

[57]  S. Hirakuni,et al.  Development of a Refrigerant Two-Phase Flow Distributor for a Room Air Conditioner , 2000 .

[58]  G. K. Nathan,et al.  Numerical and experimental studies of refrigerant circuitry of evaporator coils , 2001 .

[59]  Piotr A. Domanski,et al.  Finned-Tube Evaporator Model With a Visual Interface | NIST , 1999 .

[60]  James E. Braun,et al.  A hybrid method for refrigerant flow balancing in multi-circuit evaporators: Upstream versus downstream flow control , 2009 .