Turbulence model-free approach for predictions of air flow dynamics and heat transfer in a fin-and-tube exchanger

The heat exchanger is an important device for transporting, converting and utilizing thermal energy, which is based on heat transfer between more than two fluids. This device has been implemented in several fields such as electronics, heating, ventilating and air conditioning, automobiles, chemical and mechanical engineering, and smelting. In the recent past, energy conversion technologies have bene the focus of many as the society is embracing better ways to mitigate climate change caused by combustion of fossil fuels.

In view of this, there has been growing interests in downsizing these devices, generally from the order of 10mm of tube dimeter to the order of a few millimeters. Demand for miniaturization of products implementing heat exchangers is one of the motivations for downsizing the devices. Downsizing the heat exchangers comes with several benefits; enhanced flexibility for system optimization and instrumentation, and intensification for energy recovery processes. This would also result in increasing performance in energy saving and environmentally-friendliness.

Designing and optimizing downsized heat exchangers demand critical validation of performances with difficult measurements of pressure drops, heat fluxes and temperature differences. Implementing a numerical approach based on computational fluid dynamics can be helpful to optimize the design more effectively. In addition, several fin geometries including wavy fin, plain fin, louvered fin, corrugated fin, and fins with winglets, have been evaluated for advanced performance optimization.

Professor Ryuichi Nagaosa from the National Institute of Advanced Industrial Science and Technology (AIST) in Japan, studied the performance of a plain fin-and-tube heat exchanger based on turbulence model-free method. The plain fin geometry was chosen owing to its geometrical and mesh generation simplicity. This study also focused on the demonstration of the effectiveness as well as applicability of open-source software packages for computational fluid dynamics simulations in the areas of energy and environmental engineering. His research work is published in Energy Conversion and Management.

The author employed a turbulence model-free method to get the precise numerical predictions in a bid to avoid numerical uncertainties led by the models. It implemented three open-source software packages for performing a series of computational fluid dynamics processes. The computational fluid dynamics approach implemented 6.7×10⁶ meshes to discretize the governing equations with the help of viscous-layer meshes for scale resolution. The author varied the inlet airflow velocity from 0.25 to 8.0m/s, and compared the numerical results with laboratory results performed under equivalent experimental conditions.

The outcomes of the airflow visualizations indicated that the air flows could be categorized into 3 flow regimes; namely, unsteady flow with periodic fluctuations, turbulent flow with random fluctuations, and steady-state laminar flow. The study identified the critical Reynolds Numbers for the transitions from steady-state laminar flow to unsteady flow with periodic fluctuations at  about Re_D=4000 and from unsteady flow with periodic fluctuations to turbulent flow with random fluctuations at about Re_D=6000.

The proposed numerical work also predicted the pressure drops as well as heat transfer coefficients within an acceptable margin of error. This fact demonstrated the potential and suitability of the numerical method for practical thermal engineering problems. The proposed simulation method presented by Ryuichi Nagaosa is beneficial in introducing advanced design and optimization of heat transfer equipment with less numerical uncertainties.