Hyunjae Park

Investigation and Development of Condensation Heat Transfer Correlations for Straight and Helically Coiled Tubes

This paper introduces a set of developed condensation heat transfer correlations used in both straight and helically coiled tube systems. A literature search found that there are no applicable heat transfer correlations for two-phase flow in a helically coiled tube. As such, a study was first undertaken on condensation heat transfer in a straight tube. The investigation into condensation heat transfer for a fluid flowing in a straight tube began with a review of existing correlations. It is assumed that the annular flow is the dominant flow pattern over most of the condensing length of the tube. Depending upon the experimental methods and the theoretical and/or numerical approach, the results obtained by using the (existing) correlations show some degree of inconsistency over the ranges of parameters. Thus, it is not a simple matter to select a proper condensation heat transfer correlation for a straight tube system. This resulted in the development of generalized correlations for specific fluids such as water and R134a by employing least-squared property curves in terms of parameters such as Reynolds number based on the liquid, vapor quality and reduced pressure for each fluid considered. As a consequence, a number of plausible condensation heat transfer correlations in a coiled tube for specific fluids mentioned above are proposed employing the two-phase fluid flow Coiling Influence Factor (CIFTP ) for heat transfer obtained by using the Coiling Influence Factor for liquid and vapor (CIFl and CIFV ) respectively along with a vapor quality as a weighting parameter.Copyright

Investigation of Thermal Performance Characteristics of a Motorcycle Exhaust-Pipe System

*† ‡ § ** In this paper, an analytical model for the prediction of the thermal performance of a motorcycle exhaust-pipe system has been developed using a thermal resistance network across the exhaust-pipe tube and heat shield structure. For the selected exhaust-pipe geometric and operating conditions used in this work, the variations of the pipe and shield surface temperatures are calculated by employing proper heat transfer/thermal resistances. An experiment was performed on a bare and on insulated exhaust-pipes to develop the thermal map for the exhaust-pipe and shield surface temperature distributions for various motorcycle engine operating conditions. In order to investigate the effect of the geometric and operational parameters involved in the system, a local and an overall thermal system analysis has been performed. It was found that for the values of the parameters considered in this work, the thermal effectiveness of the existing exhaust pipe system should be improved in order to meet the rider’s thermal comfort requirement constraint.

CFD Study on Laminar Flow Pressure Drop and Heat Transfer Characteristics in Toroidal and Spiral Coil System

Most pressure drop and heat transfer correlations obtained from the toroidal geometric system have been applied to the analysis of helical and spiral tube systems. While toroidal (and helical) coils have a constant radius of curvature about the coil center point (and center-line), spiral coils have a continuously varying radius of curvature, in which the varying centrifugal forces contribute to further enhance the heat transfer (at the cost of additional pressure drop) over toroidal and helical tube heat exchangers of the same length. Due to lack of published analytical, numerical and experimental data on spiral coil systems, in this paper, the laminar flow pressure drop and heat transfer characteristics of spiral coil systems are investigated with a commercially available CFD package (Fluent 6). First, an isothermal flow CFD analysis for a toroidal coil system is performed to optimally predict the local flow field and compared with the available experimental, numerical and analytical results, in which various model assumptions and operating conditions are involved. As a consequence, the heat transfer analysis with constant wall temperature boundary condition has been performed on a toroidal coil. With the verified CFD modeling schemes such as curved geometry creation, mesh/gird density control and solution model selection, the work is extended to the spiral coil system. The effects of Reynolds number and tube diameter to coil curvature ratio on the average friction factor and heat transfer characteristics are investigated for specified coil geometries utilizing water as the heat transfer medium. The general correlations for laminar flow pressure drop and heat transfer applied in a toroidal coil system are compared with the CFD results obtained from the spiral coil systems. It was found that up to 10% of the additional pressure drop and 40% of the enhanced heat transfer characteristics are obtained from the spiral coil system over the toroidal. The heat exchanger effectiveness ratio for spirals and toroids are compared for a range of Dean number. It was found that the spiral heat exchanger effectiveness ratio was between 20 to 30 percent greater than for general toroidal heat exchanger systems.Copyright

Multidisciplinary Freshman Engineering

Abstract - A transformative first-year engineering experience has been created with the goal to immerse students in the experience of what it means to be an engineer. Students start the process of becoming an engineer, not just studying engineering. They focus on multidisciplinary engineering system investigations and discovery learning. The interaction of technology, business, human values, and complexity to achieve innovation is directly experienced by students through the application of the engineering system design process to real-world problems. The students embrace a new attitude towards learning and knowledge. They are expected to come prepared for class, ready to learn and dynamically interact. Faculties have a new attitude towards teaching, mentoring students, and addressing different learning styles: kinesthetic, aural, visual, and written. Active, integrative, inquiry- guided teaching is becoming the norm. Changing attitude and behavior is difficult for all involved, but it is happening.

CFD Investigation of Developing and Redeveloping Laminar Flow and Heat Transfer Characteristics in Coiled Tube Systems for Constant Wall Temperature Heating and Cooling: Part I — Toroid and Helical Configurations

In this paper developing and redeveloping laminar fluid flow and heat transfer performance in spiral coiled tube heat exchanger systems with specified coil-to-tube radius ratios (5 to 45), spiral pitch and inside-to-outside flow are investigated using appropriate numerical modeling techniques in the CFD package (Fluent v6.2). The CFD models employ variable thermo-physical properties in the analysis of uniform wall temperature heating and cooling of common working fluids such as air and water. The CFD models developed in Part I of this investigation are extended to analyze spiral tube configurations. Appropriate dimensionless variables used to describe the (re)developing hydrodynamic and thermal flow for coiled tube systems are defined and discussed. The spiral configuration is unique in that fully developed flow is not attained for the coil-to-tube radius ratios used in this work. In spiral configurations, the centrifugal effects are not constant as the radius of curvature is not constant. Flows within the spiral flow passages are analyzed as re-developing or re-establishing hydrodynamic and thermal flows. As with the helical flow configuration, it has been shown that in addition to the radius ratio and the Dean and Prandtl numbers, the heat transfer performance also depends upon the interactions (expansion and suppression) between the viscous and thermal boundary layers due to secondary flows caused by the centrifugal forces inherent in coiled tube systems. The predictions of the numerical simulations also showed that the local friction factor and heat transfer performance continuously vary along the length of the tube. The results obtained from this work indicate the limits of application of the friction factor and heat transfer correlations reported in the literature.Copyright

Investigation and Development of Proposed General Pressure Drop and Heat Transfer Correlations for Laminar Flow in a Toroidal Coiled Tube System

In this paper, the laminar flow pressure drop and heat transfer correlations published and applied to plain, coiled tube heat exchanger systems are extensively investigated. It was found that most correlations obtained for toroidal geometric systems have been applied to the analysis of helical and spiral tube systems. While toroidal (and helical) coils have a constant radius of curvature about the coil center-point (and center-line), spiral coils have a continuously varying radius of curvature, in which the flow does not reach a typical fully developed flow condition. The centrifugal forces, arising from the curved flow path, contribute to the enhancement of heat transfer (at the cost of additional pressure drop) over straight tube heat exchangers of the same length. In this paper, using published correlations and available experimental test data for pressure drop and heat transfer in toroidal tube systems, the proposed general correlations are developed by using a filtered-mean multiple regression method. The Coiling Influence Factors for the friction factor and heat transfer, CIFf and CIFh , respectively; defined and used in the authors’ previous works [1,2,3] it was found that the deviations between the proposed and published correlations are within about 3% for friction factor and 5–20% for heat transfer, depending on working fluid. In order to assess the validity of applying the generalized correlations developed in this work for toroidal tube systems, onto other curved tube systems, a numerical analysis of toroidal coil systems, using the commercially available CFD package (Fluent 6) has been explicitly performed. A comparison is made between the CFD result for average heat transfer (CIFh ) with that predicted by the proposed general correlation for toroidal coils and available experimental data. As an extension of this work, a comparison of curved tube over straight tube heat exchanger effectiveness is made to highlight its use as a design optimization parameter and motivation for additional coiled tube heat exchanger research.Copyright

Development of Plate-Type Heat Exchanger for Exhaust Gas Heat Recovery System

In this paper, an exhaust gas heat recuperation unit for use with a boiler is developed to improve the overall thermal efficiency of integral system. The heat recuperation unit includes a plate-type heat exchanger for exhaust gas heat recovery for exchanging heat between an exhaust gas and combustion air. A proto-type heat exchanger is designed and manufactured with a series of exhaust gas plates stacked alternatively with, and parallel to, a series of air plates. Each of the exhaust gas plates and air plates contain ridges to form a substantially sinusoidal path for directing the respective gas therealong. The combustion air flown in a countercurrent to the exhaust gas to facilitate maximum heat transfer. The heat exchanger is connected to exhaust gas inlet and outlet conduits and combustion air inlet and outlet conduits. Proper number of plates for the heat exchanger is selected to obtain laminar flow through plate flow channels, producing a low gas pressure drop in each channel. Using the proto-type heat exchanger, experimental work was primarily performed to measure temperature and pressure changes of exhaust gas and combustion air at various boiler firing conditions. These test results are compared with those obtained from numerical (CFD) and analytical works. The approximate analytical model developed in this work is used to investigate the effects of exchanger design parameters on the system performance, and eventually to develop the exchanger design curves for the optimal selection of exchanger design values.Copyright

Development of Analysis Algorithm and Computational Methodology for the Evaluation of Non-Uniform Energy Conversion System Performance: Part II — System Connectivity

It has been proposed to develop a component analysis module for each energy and heat exchanging component in a system. A system connectivity matrix has been developed to evaluate the mass and energy flow characteristics between components and the system performance. The developed component analysis modules, in conjunction with the system connectivity matrix, are exclusively used to calculate the overall and local system thermal performance. In this work, two sample energy conversion systems were selected; a vaporcompression refrigeration system and a boiler heating system. This paper examines the development of the system connectivity matrices and the corresponding iterative and/or non-iterative analysis algorithm and computational methodology. Proper mass and energy balances are formed based on the structure of the system connectivity matrix along with appropriate thermodynamic property models. The balance equations are then solved iteratively by using the developed component analysis modules until the residuals meet the convergence criteria. It was found that for the selected two systems the variations in thermal effectiveness of the two-phase heat and energy exchanging components, such as the evaporator, condenser and boiler in this work, were the main contributor of non-uniform system thermal performance.Copyright

Evaluation of Automobile Passenger Thermal Comfort Response Due to Seat Cooled by Forced Convection Heat Transfer Mode

In this paper, the existing and modified human-body heat balance equations have been used to predict the variations of automobile passenger (front and back) skin temperatures when the passenger, sitting on the (solar) heated seat, is exposed to hot vehicle air conditions in a hot summer day. The predicted passenger skin temperatures obtained by using the given vehicle air and seat conditions used in this work are used to assess the extent the passenger experiences thermal discomfort based on the thermal comfort initiation (TCI) temperature defined in this work. In order to investigate the effectiveness of the vehicle cooling seat, driven by fans installed within the seat structure, a simple one-dimensional transient analysis along with a two-dimensional numerical analysis using the commercially available CFD package (Fluent 6) has been performed to calculate the variation of seatsurface temperature with time after activating the cooling seat system. The corresponding thermal comfort initiation (TCI) time and the effective cooling region of the seat are calculated. From the case studies performed in this work, it was found that the proposed convection and conduction cooling seat system would produce seat-surface and back-skin temperature below the thermal comfort initiation temperature in about 1-2 minutes.

Numerical Investigation of Generalized Correlations for Turbulent Flow Pressure Drop and Heat Transfer Applied in Helically Coiled Tube Systems

A numerical analysis using a CFD package (Fluent v5.5) has been employed to investigate the turbulent pressure drop and heat transfer characteristics in the helically coiled tube system. The validity of using the techniques for creating coiled tube geometry and the corresponding volume mesh developed in this work is verified by explicitly comparing the numerically calculated results with those obtained from the experiments and correlations related to the straight and toroidal tubes. The authors’ previous work [5] includes the collection and summary of the general and application-specific published research and correlations. The information describing the pressure drop and heat transfer phenomena related to turbulent forced convection were combined and re-expressed into more generalized correlations using multiple linear regression techniques. In this paper, a numerical research effort using a commercial CFD package has been employed to reassess the actual phenomena with those predicted by the previously developed generalized correlations. The numerically predicted pressure drop and heat transfer coefficients at various Reynolds numbers are about 5–10% lower than those obtained by using existing generalized correlations [5]. For purposes of engineering calculations an error level of 15% or less is appropriate. The level of accuracy of the CFD modeling technique developed in this work is justified to investigate thermal-fluid phenomena in a coiled tube system.Copyright