Evaluation on the Effect of Buoyancy on the flow, heat transfer and performance of a Printed Circuit Heat Exchanger A Numerical Study

Abstract

One of the biggest threats that humanity is currently facing is the climate change. The amount of greenhouse gases emissions and specifically the amount of CO2 in the atmosphere has increased and is projected to increase even more in the next years. One possible solution is the use of supercritical CO2 as an alternative working fluid in power cycles (e.g., Rankine and Brayton cycle). Supercritical fluids operate at pressures and temperature higher than their corresponding critical pressure and temperature value. They behave as a single-phase substances with variable thermophysical properties. CO2 at supercritical conditions is denser than supercritical water and steam which can affect the efficiency and the design of multiple mechanical components. printed circuit heat exchangers due to their unique design are suitable for supercritical application. They are compact heat exchangers which can operate at high pressures and temperatures and they can achieve high effectiveness. Because of the rapid variation of the thermophysical properties, heat transfer and flow characteristics in supercritical conditions are affected. Three typical heat transfer modes can occur: 1) Normal Heat Transfer, 2) Deteriorated Heat Transfer and 3) Enhanced Heat Transfer. Furthermore, induced forces due to the density difference also known as buoyancy effects are observed in these conditions and can affect both the flow, the heat transfer of the fluid and thus the performance of the heat exchanger.
The main focus of this study is the induced buoyancy effects inside a printed circuit heat exchanger at laminar flow. Initially, a printed circuit heat exchanger is designed based on the literature. Using this heat exchanger geometry two sets of simulations are conducted in OpenFOAM. In the first set of simulations the Reynolds number and the flow thermal capacity are kept constant while the Grashof number is kept nearly constant in order to determine the dependence of Grashof number on the buoyancy effects. In the second set of simulations the Reynolds number, the flow thermal capacity and temperature difference are kept constant to evaluate the performance of the heat exchanger under real working conditions. Three geometries are under investigation (0.015/ 0.03/ 0.06 m) in three different orientations. 1) Vertical Upwards (Assisted Buoyancy), 2) Vertical Downwards (Opposed Buoyancy) and 3) Horizontal.
In the assisted buoyancy cases the induced buoyancy effects produce an M-shape velocity profile due to the acceleration of the flow near the walls and the deceleration of the flow in the centre of the channel. In this orientation the heat transfer coefficient and the Nusselt number increase and enhanced heat transfer is observed. On the other hand, in the opposed buoyancy cases the velocity near the walls decelerates and accelerates in the centre of the channel resulting to the formation of a Bell-shape velocity profile. Both the heat transfer coefficient and the Nusselt number in this orientation decreases and therefore deteriorated heat transfer is observed. It is also shown that in the opposed buoyancy cases the intense buoyancy effects can produce instabilities of the flow and recirculation zones inside the heat exchanger. Instead of the expected deteriorated heat transfer, the produced instabilities result in enhancement of heat transfer. In the horizontal cases, depending on the conditions, heating or cooling, the maximum streamwise velocity is shifted at the bottom surface (Heating conditions) and at the top surface (Cooling conditions). The formation of the secondary flow inside the horizontal heat exchangers produces an average increase of the heat transfer coefficient and the Nusselt number, thus enhancement of heat transfer.
The performance of the printed circuit heat exchanger in the three orientations is also evaluated. It is shown that buoyancy effects can directly affect the effectiveness and the performance of the heat exchanger. In the cases where enhancement of heat transfer is observed (Assisted Buoyancy and Horizontal) the heat exchanger’s effectiveness increases. For the Opposed Buoyancy cases decrease of the effectiveness is observed due to the deterioration of heat transfer inside the heat exchanger. Important parameters when investigating buoyancy effects inside the heat exchanger are the diameter of the geometry (Directly proportional to the magnitude of the buoyancy effects) and the Grashof number (Differentiates between all the cases and can be used as an indication of the importance of the buoyancy effects). In all the aforementioned cases the overall heat transfer coefficient is affected which has a direct impact on the design (Length) and the cost of the heat exchanger.