Standardized Polynomials for Fast Evaluation of Refrigerant Thermophysical Properties

Steady state and dynamic simulation and optimization are a key step in the design of heating, ventilation, air-conditioning and refrigeration (HVAC&R) systems. It is well known that the computation time in such simulations is dominated by the refrigerant thermophysical property calculations. These calculations generally involve calculating all thermophysical properties given one or two independent parameters. Refrigerant thermophysical properties are typically calculated using some fundamental equations of state (EOS). The NIST REFPROP database is an industrially accepted standard for EOS implementation. Due to the iterative nature of the EOS calculations in REFPROP, the computation time is significant and sometimes not acceptable for optimization of HVAC&R systems. In this paper, a comprehensive approach for speeding up thermophysical property calculations is presented, including the functional forms as well as the implementation aspects. A set of polynomial functional forms are presented that allow for approximation of all the thermophysical properties in all the regions for a particular refrigerant (pure fluid or a blend) of interest. The polynomials can be easily scaled to make a judicious trade-off between computation time and accuracy. Analyses for refrigerants such as R1234yf, R32, R410A, R407C and R407F are presented. Using the proposed curve fits, the saturation properties for any refrigerant can be evaluated using less than 42 floating point operations (flops) and the flash calculations with less than 300 flops per property. The mean absolute error in predicted saturation properties is 0.001% and that of flash calculations is within 0.05%. Overall the individual property calculations are 100-5000 times faster than NIST REFPROP resulting in component and system simulation speed up factor of more than 100 for refrigerant blends. The use of two standardized and scalable functional forms for approximating all properties for all refrigerants of interest facilitates easy and robust implementation on a variety of steady state and transient simulation platforms as well as on hardware since limited data needs to be stored

Airside Performance Correlations and Optimal Heat Pump Heat Exchanger Designs Based on 0.5mm-2mm Finless Round Tube Bundles

The use of small diameter tubes in air-to-refrigerant heat exchangers has significant advantages, which include increase in heat transfer coefficient, reduction in size, reduction in material or weight and reduction in refrigerant charge. However, there are no air-side correlations for small diameter tubes below 2mm in the literature. Furthermore, conventional empirical correlation development relies on testing of samples, which is inherently time consuming, expensive and has a limited range of applicability. This paper presents equations for airside friction and heat transfer characteristics for bare tube air-to-refrigerant Heat eXchangers (HX) with tube diameters ranging from 0.5mm to 2mm, and are valid for 2 to 40 rows of tubes in both staggered and inline arrangements. The correlations presented in this article are developed based on comprehensive CFD simulations for a large design space and include experimental validation. More than 80% of source data can be predicted within 10% error and more than 90% within 20% error. In this paper we use these correlations to optimize the condenser and evaporator of a 3 ton heat pump unit using R410A as the working fluid. The HX optimization framework uses a Multi-Objective Genetic Algorithm (MOGA) and an in-house HX design tool based on a segmented ε-NTU method. The optimum designs exhibit more than 50% reduction in size, and up to 50% reduction in both air and refrigerant pressure drops, compared to the baseline tube-fin HX’s using tube diameters larger than 7mm. In a system context the optimum HX’s demonstrated the ability to reduce 50% of the refrigerant charge within the HX’s and shifting majority of the system refrigerant mass to the connecting pipes. The COP is improved by 5%-7% for the same capacity

Investigation of Thermal-Hydraulic Characteristics of Pillow Plate Heat Exchangers Using CFD

The compactness and desirable thermal characteristics of plate heat exchangers have made them a strong competing heat exchanger technology in the heating, ventilating, air conditioning and refrigeration industry. The miniaturization of plate heat exchangers has become a focal point of attention in recent research. It is desirable to utilize less material and refrigerant charge to obtain the same heat transfer performance. Pillow plate heat exchangers (PPHEs) consist of wavy plates that are welded together with a certain pattern using spot welding, sealed at the edges, and then inflated in a hydroforming process. The complex wavy structure of the pillow plates creates an excellent heat transfer medium with a fully developed turbulent flow between the plates. Thus, PPHEs are used in various single-phase as well as two-phase applications in the industry. This paper presents a parametric study on the effect of critical geometrical parameters and flow conditions, on the thermal-hydraulic performance of PPHEs. The pillow surface is created using a Non-Uniform Rational B-Splines (NURBS) algorithm. The flow between two adjacent pillow plates is then investigated using computational fluid dynamics (CFD). A PPHE with optimum geometric parameters for single-phase water flow is presented

An Improved Approach for Modeling Plate Heat Exchangers Based on Successive Substitution in Alternating Flow Directions

Finite volume modeling of plate heat exchangers is challenging in terms of computation time and robustness, especially in two-phase flow due to the coupling of pressure drop and heat transfer equations. Usually the entire heat exchanger is divided into segments and solved iteratively. Many thermophysical property calculations are required for each channel in each segment which is computationally expensive, especially with a high number of iterations. This led some models to employ pre-defined databases with many correction factors to overcome slow and unstable computation. In order to overcome this, an improved approach is developed for the analysis of plate heat exchangers with multi-fluid, multi-stream, and multi-pass configurations. The model is capable of handling simultaneous phase change in all channels. In the proposed approach, the fluid properties are propagated in one flow direction, while the fluid properties in the opposite direction are calculated in a given iteration. These operations are switched in the next iteration. The convergence of this approach is verified numerically. This approach shows significant improvement in computation speed and robustness compared to current models. This approach solves 6-12 times faster in terms of number of iterations required to solve a plate heat exchanger and more stable especially with a lower number of slices compared to existing model. The model is validated against 150 in-house experimental data points for single phase water, two-phase ammonia, and R22 boiling, and two-phase R134a, and R410A condensation. Overall, the model predicts heat capacity within 5%

Micro combined cooling and power

Part of: Thermally driven heat pumps for heating and cooling. – Ed.: Annett Kühn – Berlin: Universitätsverlag der TU Berlin, 2013 ISBN 978-3-7983-2686-6 (print) ISBN 978-3-7983-2596-8 (online) urn:nbn:de:kobv:83-opus4-39458 [http://nbn-resolving.de/urn:nbn:de:kobv:83-opus4-39458]Thermally driven chillers can be driven by waste heat from prime movers (engines, turbines or fuel cells) to form combined cooling and power (CCP) systems. In this chapter, a method of matching chillers to prime movers is presented. CCP configurations with and without backup cooling are described, along with first-order estimates of the energy efficiency of each combination of configuration and prime mover. Some experimental results for micro combined cooling heating and power (CCHP) are presented. Based on the analytical and experimental work, it is concluded that CCP and CCHP performance depend heavily on the choice of prime mover. CCP systems based on fuel cells can use less energy than grid-driven electrical cooling systems. CCP with combustion-based prime movers has potential to save energy in off grid applications

Optimization of a Residential Air Source Heat Pump using Heat Exchangers with Small Diameter Tubes

Heat exchangers play significant role in refrigeration and air conditioning systems. Ongoing research aims to improve, or at least maintain, the system performance while reducing the size, weight and cost of the heat exchangers. This in turn leads to lower system refrigerant charge and reduced environmental impact. Using heat exchangers with small tube diameters (less than 5 mm) instead of large tube diameters has been shown to be a promising solution to meet the aforementioned goals. However, shifting towards mall tube diameters requires in-depth analysis and optimization of several heat exchanger design parameters. This paper presents a multi-objective optimization of a residential air source heat pump system using genetic algorithms with a particular focus on the use of small diameter tubes in the heat exchangers. The objectives are to minimize the heat exchangers’ cost and maximize the system performance. The goal of this study is to determine the potential material savings and cost reduction when using tube diameters between 3 mm and 5 mm in the heat exchangers. In addition to the tube diameters, multiple fin types, tube spacing and fin densities are also investigated. The optimization is carried out for R-410A, and a lower-GWP alternative, R-32. The system utilizing the improved heat exchanger designs has a cost reduction of 50% in comparison to the baseline system. Also, the improvements in the system’s COP and the system charge reduction are around 20% and 35%, respectively.

Steady State Modeling of Advanced Vapor Compression Systems

The use of heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems is always increasing. This is because the HVACR systems are necessary for food production and ability to inhabit buildings that otherwise would be inhabitable. Thus, there is continued research focused on improving the efficiency and reducing the negative environmental impact of these systems. The basic vapor compression cycle (i.e., evaporator, condenser, expansion device and compressor), which is still the main underlying HVAC&R technology worldwide, has already reached its limits and researchers are investigating more creative and complex cycles to improve capacity and efficiency. This motivates the development of an enhanced general vapor compression system steady state solver. Steady state simulations require less time than transient simulations, and are used in system design optimization and cost minimization for given performance. This paper presents a comprehensive vapor compression system steady state solver which has several novel features compared to the existing solvers. Firstly, this proposed solver is capable of simulating large number of different designs of vapor compression systems. This includes arbitrary system configurations, multiple air and refrigerant paths, and user defined refrigerants. The solver uses a component-based solution scheme in which the component models are treated as black box objects. This allows a system engineer to quickly assemble and simulate a system where in the component models and performance data comes from disparate sources. This allows different vapor compression systems design engineers, and manufacturers to use the solver without the need to expose any possible confidential component data. The solver is validated using a vapor injection heat pump system with a flash tank and the preliminary modeling results match the experimental results within 10% accuracy. This heat pump system model is also tuned in order to improve the validation accuracy. A parametric case study for a variable refrigerant flow (VRF) system is presented as well to demonstrate the applicability to larger systems

Wavy Fin Profile Optimization Using NURBS for Air-To-Refrigerant Tube-Fin Heat Exchangers with Small Diameter Tubes

The major limitation of any air-to-refrigerant HX is the air side thermal resistance which can account for 90%, or more, of the overall thermal resistance. For this reason the secondary heat transfer surfaces (fins) play a major role in these HX’s by providing additional surface area. Many researchers extensively investigate how to improve the performance of fins. The most common passive heat transfer augmentation method applied to fins uses surface discontinuity; providing an efficient disruption-reattachment mechanism of the boundary layer. Such approach is leveraged by louvers, slits and even vortex generators. In some applications, however, these concepts are not adequate especially when there is high fouling or frosting, which is the case of many HVAC&R systems including heat pumps for cold climates. In such cases a continuous fin surface is required, which can usually be plain or wavy. The latter provides larger surface area and can induce turbulent flows improving the heat transfer. Normally the wavy fins are either a smooth sinusoidal or Herringbone profile, longitudinal to the airflow direction. In this paper we propose a novel wavy fin design method using Non-Uniform Rational B-Splines (NURBS) on both longitudinal and transverse directions. In this method the fin surface is subdivided in to 1 x n identical cells with periodic boundaries. The horizontal and vertical edges independently describe a NURBS curve on separate planes with the third spatial direction. The tools used in this work include automated CFD simulations, metamodeling and Multi-Objective Genetic Algorithm (MOGA). The analysis comprises of optimizing all wavy fin types, both the conventional ones and the novel designs presented in this paper, and compare their performance and compactness while fixing hydraulic diameter and Reynolds numbers. In conclusion, design recommendations for made for the use of the proposed novel fins.

A Model for Performance Prediction of Brazed Plate Condensers with Conventional and Alternative Lower GWP Refrigerants

Plate heat exchangers are used in a wide variety of applications from air-conditioning, refrigeration, food processing, and chemical industry to energy generation systems. Plate heat exchangers are favored because of their compactness, flexible thermal sizing, close approach temperature, pure counter-flow operation, and enhanced heat transfer performance. This paper presents a literature review of available correlations for heat transfer and pressure drop calculations during condensation in brazed plates. Condensation heat transfer in plate heat exchangers can be preliminarily evaluated using the classic Nusselt equation for laminar film condensation on a vertical plate. However, condensation performance is influenced by many factors such as fluid properties, plate geometry, and mass flow rate, and therefore, it is difficult to obtain an ideal correlation which accounts for all these factors. The heat transfer coefficients of different refrigerants, including alternative lower GWP refrigerants R32, D2Y60, and L41a, are computed using heat transfer correlations found in literature. Generally, R32 shows the most favorable heat transfer performance, followed by L41a. An overview on an already existing model that is capable of predicting the performance of plate heat exchangers with generalized multi-fluid and multi-pass configurations is also presented. The model is validated against experimental data for water-to-R134a condenser. A total of sixteen experimental datasets are used. The heat capacity predicted by the model is within ±5% of the measured heat capacity, while most of the predicted outlet temperatures are within ±2 K of measured values