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Development of Polymer Heat Exchangers : Abstracts

The use of polymer tubes for heat exchanger tube bundles is of interest in many applications where corrosion, mineral build-up and/or weight are important. The challenge of overcoming the low thermal conductivity of polymers may be met by using many small-diameter, thin-walled polymer tubes and this route is being pursued by industry. We propose the use of unique shaped tubes that are easily extruded using polymeric materials. The shaped tubes are streamlined to reduce form drag yet the inside flow passage is kept circular to maintain the pressure capability of the tube. Special treatment is required to predict convective heat transfer rates because the temperature distribution along the outer surface of the shaped tubes is non-uniform. The average forced convection Nusselt number correlations developed for these non-circular tubes can not be used directly to determine heat transfer rate. In this paper, heat transfer rates of shaped tubes are characterized by treating the tubes as a base circular tube to which longitudinal fin(s) are added. Numerical solution of the non-linear differential equation balance on the fin provides the surface temperature distribution and a shaped tube efficiency, which can be used in the same manner as a fin efficiency to determine the outside convective resistance. The approach is illustrated for three streamlined shapes with fins of lenticular and oval profile. The presentation of the results highlights the effects of the geometry and the Biot number on the tube efficiency and heat transfer enhancement. Convective heat transfer is enhanced for the oval shaped tube for 2000 < Re < 20,000 when Bi < 0.3. For polymeric materials, the Biot number in most applications will be greater than 0.3, and adding material to the base tube reduces the heat transfer rate. The potential benefit of reduced form drag remains.

Mechanical Analysis of Streamlined Tubes with Nonuniform
Wall Thickness for Heat Exchangers
Zhihua Li, Susan C. Mantell, Jane H. Davidson

An approach to select the tube wall thickness distribution of streamlined tubes intended for use in heat exchangers is developed. The goal is to retain a streamlined outer profile (resist deformation) and prevent strain failure due to the applied internal pressure. The effect of the tube wall thickness distribution on heat transfer is also considered. The strain is calculated as a function of several dimensionless geometric ratios and the ratio of the internal pressure to material modulus. Using the finite element method, a set of dimensionless design curves is created for elliptical tube geometries. From these curves, a range of possible materials and tube geometries can be selected that meet a specific strain limit. To illustrate the approach, structure-satisfied elliptical designs are selected and their thermal performance is evaluated for an automotive radiator and an automotive charge air cooler made of polymeric materials. The same method can be extended to tubes of other shapes and materials.