Convective heat transfer in cross-corrugated triangular ducts under uniform heat flux boundary conditions

https://doi.org/10.1016/j.ijheatmasstransfer.2010.09.010Get rights and content

Abstract

Cross-corrugated triangular ducts provide high heat mass transfer capabilities in membrane based air-to-air heat mass exchangers. The mixing effect would intensify the convective heat mass transfer coefficients on membrane surfaces. In this study, the fluid flow and convective heat transfer in a cross-corrugated triangular duct under uniform heat flux boundary condition is modeled and experimentally studied. A low Reynolds number kω (LKW) turbulence model is employed to account for the turbulence in the flow. Heat transfer experiments and high speed hot wire anemometry technology are used to validate the model. The transitional behavior of fluid flow in the duct is disclosed by velocity measurements and Fourier transforms. Correlations are provided for estimation of the pressure drop and the mean Nusselt numbers under uniform heat flux boundary conditions. The established correlations can be extended to estimate the convective mass transfer coefficients through heat mass analogy.

Introduction

Indoor air quality and thermal comfort have become a hot topic in recent years. Fresh air ventilation is efficient in maintaining a healthy building. Membrane-based air-to-air heat mass exchangers have attracted much attention for conditioning fresh air while saving energy [1]. As seen in Fig. 1, the device is just like an air-to-air sensible heat exchanger. But in place of traditional metal heat exchange plates, hydrophilic polymer membranes, which can transfer both heat and moisture simultaneously, are used as the heat and mass transfer media. The device has many virtues like it is stationary, compact, and easy to construct. However, practical application until now is still scarce. The reason is that heat and mass transfer in the unit is slow, which limits their market penetrations. Both the heat and mass transfer in the duct and in the membrane itself are important. There have been many studies concerning heat mass transfer intensification in membrane [2], [3], [4]. This study will switch the focus to duct side transport intensification. Mass diffusion in the membrane itself is not considered by assuming uniform flux boundary conditions on membrane duct surfaces.

Besides heat and moisture diffusion in membranes, convective heat and mass transfer in the exchanger duct is of vital importance. To intensify convective heat and mass transfer, an exchanger structure, i.e., cross-corrugated triangular ducts, has been proposed [5], [6], [7], [8]. The concept is shown in Fig. 2. Flat membrane sheets are corrugated to form a series of parallel equilateral triangular ducts. Sheets of the corrugated plates are then stacked together to form a 90° orientation angle between the neighboring plates, which guarantees the same flow pattern for both fluids. The membranes are very thin and soft, which requires plastic frame cases to support them. Consequently, with a pre-designed plastic frame, triangular shaped duct walls are formed to construct the required geometry [8]. The structure gives better heat mass transfer. This efficiency improvement is attributed to the pattern of flow that undergoes abrupt turnaround, contraction, and expansion.

Convective heat and mass transfer coefficients in the duct are necessary for the estimation of exchanger performance. Literature review found that there have been several studies in this field. Scott and co-workers [5], [6] measured the convective mass transfer coefficients for a liquid solution in a cross-corrugated duct. Other investigators in heat and/or mass transfer in corrugated triangular ducts include Dong et al. [9], Gupta et al. [10] and Lee et al. [11]. Regretfully, the fluid flow in the duct and the convective heat and mass transfer coefficients for gas flow were not mentioned. In recent years, current authors have studied the fluid flow and heat transfer in this structure [7], [8]. However previous studies were for heat transfer under uniform temperature boundary conditions. Further, these studies were solely numerical and no experimental data was reported. It should be noted that the boundary conditions for a counter flow heat mass exchanger can be approximated by a uniform heat and mass flux condition. The fluid flow and transport data in the duct under such conditions are highly desired. This is the object of this study.

Section snippets

Heat transfer

A test rig, as shown in Fig. 3, is used to perform heat transfer and pressure drop experiments. It is an open circuit system comprising of five main components: a variable speed blower, a wind tunnel, an upstream section, a test section, and a down-stream section. Air conditioned indoor air is supplied to the tunnel by a variable speed blower. The volumetric air flow rates can be adjusted to have different Re numbers from 50 to 30,000. The low speed wind tunnel is to ensure a continuous, steady

Governing equations

In the experiments, the inlet velocities are from 1 to 10 m/s. Under these conditions, the Re numbers are from 321 to 3210. The flow in the duct is neither pure laminar nor fully developed turbulent. As previously reported [7], the flow is mostly in the transitional flow regime.

Usually, as fully turbulent flows, transitional flows are modeled with turbulence models. The equations describing the fluid flow and heat transfer are transport equations for the continuity, momentum, and energy, which

Transitional behaviors

The velocity in the duct shows some turbulent behavior even below Re = 2300. The velocities at the center of the duct on cross section x = 4.5 are analyzed. The oscillations of velocity components (streamwise, spanwise, and normal) at the monitoring point (center on face x = 4.5) under three frontal velocities are shown in Fig. 8 (Ui = 2 m/s, Re = 641), Fig. 9 (Ui = 4.0 m/s, Re = 1282) and Fig. 10 (Ui = 6.0 m/s, Re = 1923), respectively. As seen, the higher the frontal velocities, the larger the scale of

Conclusions

Fluid flow and convective heat transfer in a cross-corrugated triangular duct under uniform heat flux boundary conditions are investigated. Detailed heat transfer experiments and hot wire anemometry technologies are performed to disclose the transitional behavior and heat transfer performances in the geometry. A three-dimensional numerical study is also performed to aid in the analysis. The low Reynolds number kω model is used and validated in the analysis for Re range of 500–5000. The results

Acknowledgements

The Project is supported by Natural Science Foundation of Guangdong Province, No. 8151064101000041, and the Specialized Research Fund for the Doctoral Program of Higher Education, No. 20070561034. It is also partially supported by National Natural Science Foundation of China, No. 51076047, and The Fundamental Research Funds for the Central Universities, 2009ZZ0060.

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