CFD-Based Correlation Development for Air Side Performance of Wavy Fin Tube Heat Exchangers using Small Diameter Tubes

A great deal of effort is spent on the design and optimization of air-to-refrigerant heat exchangers in HVAC&R applications. One path towards improving their performance is the transition to smaller hydraulic diameter flow channels. In particular, tube diameters below 5mm need to be investigated. The in-tube refrigerant flow characteristics are well understood for small diameter tubes and reliable heat transfer and pressure drop correlations are available in the literature. On the air side, however, most of what is available in the literature has no, or very limited, applicability to small tube diameter heat exchangers. In these situations numerical methods such as CFD are commonly employed in the performance evaluation of tube and fin surfaces. Although CFD has been a powerful and reliable tool it is still computationally expensive if used for evaluating a large number of parameterized geometries. This work presents new CFD-based correlations for wavy-finned tube heat exchangers with tube diameter ranging from 2mm to 5mm. Herringbone wavy fin profiles are analyzed in this study. The methodology implemented in this work consists of analyzing air-side heat transfer and pressure drop characteristics by using Parallel Parameterized CFD (PPCFD) which helps in reducing engineering time significantly. The CFD models are verified using the Grid Convergence Index (GCI) method. The verified CFD model are then used to generated air-side performance data for a wide range of geometrical parameters such tube diameters, spacing and fin density and operating parameters such as tube wall temperature and inlet air state. The resulting data is reduced into correlations that can be easily implemented in various heat exchanger analyses tools. As new experimental data becomes available, the correlations will be updated. In the meanwhile, researchers and engineers can use these correlations for evaluating the performance of small tube diameter heat exchangers

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.

Performance Evaluation Criteria & Utility Function for Analysis of Compact Air-to-Refrigerant Heat Exchangers

The Performance Evaluation Criteria (PEC) constitutes a set of metrics that quantifies geometrical characteristics, and performance of a Heat eXchanger (HX) under design and off-design conditions. There is a vast literature describing different PEC methods and it can be separated into two main categories: energy-based and entropy-based metrics. The first include all metrics that directly measure the thermal-hydraulic performance and geometry features such as surface density, face area and aspect ratio. The second approach entails a more fundamental perspective by employing the second law of thermodynamics to determine the best and worst heat transfer surfaces in terms of entropy generation and/or exergy destruction. Usually the second approach brings a broader perspective when evaluating the HX in a system context. From a design viewpoint the choice of PEC is really a matter of preference, as long as the problem specifications are being met. A more challenging task, however, is the selection of the best HX amongst multiple alternatives. More recently, with the great advances in computational tools, a large number of novel HX concepts and multi-objective optimization (MOO) studies are being undertaken. When performing MOO analysis one must be able to understand why did the optimizer selected those optimum designs, and be able to know which one to select from a set of Pareto Optimal designs. In other words, from a decision-making viewpoint the use of PEC’s is less trivial. In this paper we provide a brief review of the available PEC in the literature for HX design. Additionally we present a set of PEC metrics that should be used for selecting a HX amongst multiple optimum designs, sized to perform the same job, i.e. same heat load capacity and fluid states. We developed a utility function, using such metrics that will better assist the decision-maker in selecting the best alternative. This utility function was specifically developed for single-phase air-to-refrigerant HX application, and applied to a case of study consisting of multiple optimum Pareto sets for different surfaces. Additional CFD analysis is also carried out for completeness and to illustrate the underlying physics of the airflow on different surfaces that lead to the differences in performance between different surfaces

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

CFD-Based Correlation Development For Air Side Performance Of Finned And Finless Tube Heat Exchangers With Small Diameter Tubes

Air-to-refrigerant heat exchangers are a key component in air-conditioning and heat pump systems. A great deal of effort is spent on the design and optimization of these heat exchangers. One path towards improving their performance is the transition to smaller hydraulic diameter flow channels. This is evident by the recent introduction of microchannel heat exchangers in the stationary HVAC market. Systematic analyses demonstrates a great potential for improvement in terms of size, weight, refrigerant charge and heat transfer performance by employing small diameters in tube-fin heat exchangers. In particular, tube diameters below 5mm need to be investigated. It is known that as tube size reduces, at some point, fins are no longer required and the heat exchanger then comprises of just bare tubes. The in-tube refrigerant flow characteristics are well understood for small diameter tubes and accurate heat transfer and pressure drop correlations are available in the literature. On the air side, however, most of what is available in the literature has no or very limited applicability to small tube diameter heat exchangers. In these situations numerical methods such as CFD are commonly employed in the performance evaluation of tube and fin surfaces. Although CFD has been a powerful and reliable tool it is still computationally expensive if used for evaluating a large number of parameterized geometries. This work presents new CFD-based correlations for finned and finless tube heat exchangers for tube diameter ranging from 2mm to 5mm. The methodology implemented in this work consists of analyzing air-side heat transfer and pressure drop characteristics by using a method called Parallel Parameterized CFD (PPCFD). PPCFD allows for fast, automated parametric CFD analyses of various geometries with topology change and reduces the engineering time significantly. The CFD models are verified using uncertainty analysis methods and by comparing the predictions against available experimental data. The validation study shows that CFD can predict the air side performance within 10% as compared to experimental data and can reproduce the performance trends very well. The verified CFD model are then used to generated air-side performance data for a wide range of geometrical parameters such tube diameters, spacing and fin density and operating parameters such as tube wall temperature and inlet air state. The resulting data is reduced into correlations that can be easily implemented in various heat exchanger analyses tools. As new experimental data becomes available, the correlations will be updated. In the meanwhile, researchers and engineers can use these correlations for evaluating the performance of small tube diameter heat exchangers

Performance Analysis of Ejector Cycles for Separate Sensible and Latent Cooling in Air Conditioning

While the overall system efficiency of split air conditioning (AC) systems has improved over the last three decades, residential air handling units (AHUs) used in those systems have essentially stayed the same in size, shape, form, and efficiency. Incremental improvements have been made to AHUs to address safety, functionality, and energy-efficiency concerns, however, their overall structure has remained the same. A promising technology that addresses fundamental challenges with conventional cycle efficiencies are ejector-based cycles, more commonly employed in refrigeration applications, but with great potential in AC systems as well. An ejector employed as an expansion device can recover expansion losses, boost pressure, and facilitate a dual evaporator system. This paper presents four categories of ejector enhanced vapor compression cycles (VCCs) leading to seven potential system concepts: standard two-phase ejector, two variants of condenser outlet split (COS), diffuser outlet split (DOS), and three variants of separator outlet split (SOS). The concepts were investigated via numerical model studies and two promising ejector enhanced cycles for a residential AC application emerged: COS and DOS. The COS and DOS ejector enhanced cycles improved seasonal energy efficiency ratio (SEER) by 4%–8% above a 15 SEER baseline AC system and improved the total coefficient of performance (COP) by 9%–11%. With the COS or DOS ejector enhanced cycles, losses quantified by exergy destruction were reduced by up to 18%

A Study on Computational Cost Reduction of Simulations of Phase-Change Material (PCM) Embedded Heat Exchangers

Thermal storage can be implemented using Phase-Change Materials (PCM), which absorb significant latent heat with a relatively small temperature change. PCM phase-change processes are transient and are driven by thermal diffusion and natural convection – the latter, especially for melting process. Modeling and simulation of PCM heat exchangers (HX’s) is typically computationally intensive due to the relatively complex time-dependent physics. Most of the PCM modeling work in the literature uses high-order modeling tools such as Computational Fluid Dynamics (CFD) and Lattice-Boltzmann Method (LBM). For design purposes, the existing PCM modeling approaches are not practical, limiting researchers in their ability to investigate new ideas and different PCM’s with faster turnarounds. This paper presents a study investigating the reduction of computational cost of PCM embedded HX’s CFD models by evaluating the feasibility of spatial reduction without losing accuracy. The analysis consists of comparing full and partial domain under full melting conditions. The subject of this study is a single straight tube with circular transverse fins in the vertical orientation, using PCM’s with 35oC nominal melting temperature. Different tube and fin dimensions are investigated. Results indicate that the reduced domain reproduces -in half the run time -the same behavior as the full domain since the buoyancy effects are localized and patterned. The outputs from the partial domain simulation were used to build a non-general correlation for the PCM heat transfer characteristics and demonstrated how it can be implemented in a Finite Control Volume Reduced Order Model (ROM). The ROM can accurately reproduce the CFD simulations at 4 to 5 orders of magnitude faster

Numerical Study And Validation Of Melting And Solidification In PCM Embedded Heat Exchangers With Straight Tube

Latent heat thermal energy storage (LHTES) systems have shown great potential to enable reliable use of renewable energy and load shifting. LHTES offer high storage density and release energy at near constant temperature because of its use of phase change materials (PCMs). The cylindrical PCM heat exchangers (PCMHX) are one of the most used technologies due to their simplicity. Numerical models for such PCMHX enable engineers to estimate their performance for different design parameters and operating conditions without having to test them all. However, modeling the phase change phenomena can be challenging. To better understand the difficulties involving accurate modeling of PCMHX, a cylindrical latent storage unit filled with PCM and water as in-tube heat transfer fluid (HTF) is numerically investigated. This paper presents a study based on a 2D-axisymmetric model of a straight tube embedded in PCM in a cylindrical container. CFD is used to study the charging (melting) and discharging (solidification) phenomena. The models are validated against experimental and numerical data from the literature. The predicted local PCM temperature profile over time agrees within 2K compared to the experimental values. The paper also presents a simple method to estimate the melting and solidification phase change temperature range from limited data provided by PCM manufacturers

AIRSIDE PASSIVE HEAT TRANSFER ENHANCEMENT, USING MULTI-SCALE ANALYSIS AND SHAPE OPTIMIZATION, FOR COMPACT HEAT EXCHANGERS WITH SMALL CHARACTERISTIC LENGTHS

The study of compact heat exchangers (HX) is a very common, although broad topic that draws interest from many engineering applications. Most technologies contain at least one HX serving as a fundamental component for the proper system functioning. The rapid worldwide population growth, increasing demand for energy resources, widespread environmental concerns, space exploration efforts and economy are all good reasons for developing smaller, lighter and more efficient HX’s. This research sheds the light on the next generation of heat exchangers, with a focus on air-to-fluid applications. For incompressible flows and low-pressure applications, the HX’s airside thermal resistance is the major limitation to overall thermal conductance. On conventional surfaces fins are required, but bring many drawbacks. Among these include being prone to fouling/frosting, reduced heat transfer coefficient, higher friction resistance, and more material consumption. Tubes by nature provide more valuable heat transfer than do fins; there is little focus on tubes in the literature. The first objective of this work is to discuss the fundamental aspects of primary (tubes) and secondary (fins) surfaces, with the aid of numerical analyses. The latter demonstrates how the reduction of characteristic length and novel shapes impact surface performance and compactness of finless and finned tubes. A further discussion is presented arguing that conventional fin concepts are not always beneficial. The second objective of this work entails developing a comprehensive multi-scale analysis with topology and shape optimization methodology leveraging automated CFD simulations and approximation assisted optimization. Novel finless air-to-fluid HX concepts were developed, for single-phase and two-phase applications, and achieved more than 20% reduction in size, 20% better performance and 20% less material than state-of-the-art HX’s including microchannel HX’s. Two prototypes (one manufactured in metal 3D printing) were tested in an in-house wind tunnel. The numerical predictions agree with the experimental results in less than 5% deviation for total capacity, 10% for airside heat transfer coefficient and 20% for air pressure drop. Finally, the last objective is to present the development of robust and computationally inexpensive tools that can accurately predict CFD simulation responses for conventional tube and fin surfaces using small diameter tubes (<5.0mm)