CFD Simulation of the Thermal Performance of a Parallel Counter-Parallel Flow Heat Exchanger for the Treatment of Hypothermia
Creators: name: Heller, Alex 
Publisher: name: University of Nevada, Las Vegas
Year: 2014

description: Hypothermia is a life-threatening condition. Currently, active warming methods are the most effective treatment for dysthermic patients. The aim of this study is to investigate the use of computational fluid dynamics (CFD) in evaluating the thermal performance of a parallel/counter-parallel flow heat exchanger used as part of a fluid warmer to treat Hypothermia. The 3D model of the heat exchanger is divided into three regions; Infusate (fluid to be heated), Hot Water (heating fluid), and a Solid Region (wall). At the end of the heat exchanger, an elbow section is used to create the counter-parallel flow arrangement specific to this design. 
The primary focus of this study involves evaluating heat transfer between the Infusate and Hot Water regions. Several simulations were performed for varying heat exchanger lengths (0.6, 1.2, 1.8, and 2.4m). The current CFD predicted values were compared to previously collected experimental data. In the experimental set-up, the outlet temperature was evaluated using a center-point temperature probe. The current CFD study evaluated the outlets in terms of mean bulk temperature to better characterize the thermodynamic average with respect to fluid flow. Despite this difference, the CFD results of the Infusate outlet temperatures were within 20% of the previously published experimental values. Using a center-point temperature probe, the CFD simulations were within 8% of the experimental values. It was concluded that the CFD model accurately represented the thermodynamic characteristics of the heat exchanger and can be used for future design purposes. 
The Hot Water region features a unique geometric variation on the traditional concentric annulus; a separation along the mid-plane confines the flow to have the area of a true concentric annulus. As such, CFD was used to investigate the thermalhydraulic effects within the Hot Water region (semi-annulus). Correlations were developed to predict the hydrodynamic and thermal developing lengths within the Hot Water region. These correlations were determined for developing flow at the inlet and after the elbow sections of the Hot Water region. CFD simulations of the Hot Water region demonstrated increasing hydrodynamic and thermal developing lengths for increasing Reynolds Number under laminar and turbulent flow. Increased mass flow rates produce increased forces within the flow area, requiring increased axial length for thermal and hydrodynamic profiles to stabilize. 
Developing effects within a concentric annulus has been addressed in the literature; however the flow characteristics within the proposed semi-annulus are not as well understood. A comparative study was performed evaluating the studied Hot Water semi-annulus against a true concentric annulus. The CFD developed hydrodynamic entrance length correlation for the Hot Water region (semi-annulus) under laminar flow was compared to a known entrance length correlation for a true concentric annulus. Due to the separation along the mid-plane of the Hot Water region, the flow area and wetted perimeter of a true concentric annulus is expected to be greater than that of the semi-annular geometry. Greater flow interaction with the wall in the semi-annulus increases the viscous drag on the fluid, resulting generally in a lower non-dimensionalized developing length as a function of Reynolds Number. 
Pressure drop within the Hot Water region was also evaluated and compared to known properties of a true concentric annulus within the fully developed region. Under both laminar and turbulent flow, the studied semi-annulus of the Hot Water region demonstrated greater pressure drop than a true concentric annulus with similar dimensions and flow conditions. The elbow section of the Hot Water region produced a significant pressure drop, which may be due to the abrupt change in flow direction resulting in increased centrifugal forces and recirculation zones. 
The thermal and hydrodynamic properties revealed in the CFD simulations can be used to improve future design considerations, which may lead to improved Hypothermia treatment protocols and patient care.