Heat Transfer Enhancement Using Shaped Polymer Tubes: Fin Analysis

Abstract

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 nonuniform. The average forced convection Nusselt number correlations developed for these noncircular 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 an energy 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 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.

Recent research developments in polymer heat exchangers – A review

“…In order to reduce the conductive resistance of the polymer wall and maintain acceptable pressure drops, researchers have focused on the efforts to develop polymer tube bundles with small diameter and thin wall tubes. The fluid dynamic and thermal performance of using the unique shaped polymer tubes in bundles comprised of small diameter tubes (as shown in Figure 23) was conducted by Li et al 88 . The outer surface of the tube was streamlined into oval shapes to reduce form drag and the inner flow passage was kept circular to maintain pressure capability.…”

“…The numerical analysis revealed that such geometry changes improved convective heat transfer with Re numbers in the range of 2000 to 20,000 and a Biot number less than 0.3. Figure 23 Tube bundle of 'shaped' polymer tubes 88 Park and Jacobi 89 proposed an analytical method to model the polymer heat exchanger tube bundle geometry. The tube core mass/ volume ratio, facial area ratios were tuned according to various heat transfer rate, pumping rate and pumping power.…”

Heat Transfer Performance of an Edge-Shaped Finned Tube

“…Although finned tubes employing the third-generation heat transfer elements with various geometrical shapes have been exploited, few data related to their condensation or boiling heat transfer performances are seen [9,16]. Furthermore, studies on heat transfer performance of tubes that integrate multiple kinds of the third-generation heat transfer technologies are absent in open literature.…”

Polymeric Heat Exchangers: Effect of Chemistry and Chemical Composition to Their Performance

“…Polymers are large molecules made up from the repetition of subunits (monomers). In general, polymers have lower thermal stability and conductivity [1], problems with creeping, degradation, and lack of mechanical properties relative to metals. However, they have already been used as heat exchangers for many years due to benefits such as corrosion resistant, lightweight, low cost as compared to exotic metals, easy to manufacture, ability to handle both liquids and gases, and inherent resistance to fouling [2][3][4].…”