Conjugated Heat Transfer, Elliptical Finned Tube Heat Exchanger Vs. Circular Fin

Description

Introduction

The use of heat exchangers in the oil and gas industry and power plants is prevalent. Finned tube heat exchangers are used to cool the desalinated crude oil. These heat exchangers ameliorate heat transfer by increasing the effective surface area due to the high fins and creating more turbulence in the flow. Fins are used when the working fluid in contact with that part has a lower heat transfer coefficient. The following figure shows an example of this type of heat exchanger.

Problem Description

In this project, the effect of cross-sectional change (with the constant area) on the rate of improvement of heat flux passing through the finned tube heat exchanger was compared by investigating conjugated heat transfer. Ansys Fluent software was used for the simulation. Two circular and elliptical sections with equal cross-section areas are examined for the internal fluid section. One row of this type of finned tube heat exchanger was modeled to reduce the computational costs. The hot inlet fluid was crude oil with a temperature of 200 ° C and a velocity of 0.3 m / s. Air fluid with a temperature of 25 ° C and velocity of 2 m / s was used for the external flow section. The figure below shows the computational domain of the simulation.

Finned Tube Heat Exchanger Geometry & Mesh

The geometry was designed using the Design modeler module (figure below). Its geometric specifications consist of a pipe with a diameter of 10, a thickness of 1 and a length of 100 cm, and square fins with a side of 30 cm. For the elliptical case, the length of the large-diameter was 10, and the small diameter was 2.5 cm. The thickness of the fins was 0.5 cm and at a distance of 20 cm from each other, which makes a total of 10 fins.

For grid generation, unstructured mesh with about 5 million elements in the Ansys meshing module was used. In addition, a boundary layer mesh was used near the walls to satisfy the Y-Plus number condition of the enhanced wall treatment. The following figure shows the meshing geometry.

Solver Setting

Fluent software was used to solve the governing equations numerically. The problem is analyzed steady, using the pressure-based method and neglecting the gravitational effects.

Boundary conditions and Solution methods

Also, The table below shows the characteristics and values of boundary conditions, along with the models and hypotheses.

Finned Tube Heat Exchanger Results

In this section, by examining the heat flux passing through the finned tube wall for both circular and elliptical cross-sectional surfaces, it was observed that the cross-sectional change from circular to elliptical increased the heat flux passing through the heat exchanger(Figure below).

Circular Finned Tube Heat Exchanger Heat Flux

Elliptical Finned Tube Heat Exchanger Heat Flux

The main reasons for this improvement in heat transfer are the increase in the effective surface contact with the airflow due to the elliptical cross-section and the increment in the intensity of the flow turbulence when using the elliptical cross-section.

Also, by observing the temperature contours on the wall of the heat exchanger, it was found that a higher temperature distribution was created for the ellipse section in the fins.