Numerical investigation of the effect of surface roughness on flow and heat transfer characteristics of single sphere particle in supercritical water
Introduction
Supercritical water fluidized bed (SCWFB) reactor [1] is a novel reactor which can convert coal into hydrogen-rich gas efficiently and cleanly [2], [3], [4]. At present, the studies on SCWFB were mostly aimed at investigating the effects of reaction conditions [4], [5] (time, temperature, pressure, flow rate, reactants and catalysts) on gasification characteristics. However, because of the special properties of supercritical water (SCW) such as low viscosity, high diffusivity and high reaction efficiency [6], [7], the theories for conventional fluidization reactor may be not fitted to predict two phase flow and heat transfer characteristics in SCWFB [8]. The lack of understandings of two phase characteristics in SCWFB may affect the design and operating condition optimization of the reactor [9]. While it is difficult to observe related phenomena by experimental method due to the limitation of high pressure and temperature. Thanks to the rapid development of computational fluid dynamics, numerical simulation has been used as an effective tool to study on the flow and heat transfer phenomena at different scales [10], [11], [12] without limitation of experimental approaches.
In recent years, numerous studies have focused on studying the effect of particle shape in SCWFB. Lu et al. [13], [14] built the correlations of drag coefficient and Nusselt number of single sphere particle in SCW. They mainly considered the impact of severe property change in the pseudo-critical zone. Jin et al. [15], [16] investigated the drag coefficient of single sphere and cylinder particle in SCW. They found that there was no significant difference for drag coefficient of particles between ambient condition and supercritical condition under the assumption of the constant fluid properties. Zhang et al. [9], [17] proposed the correlations of drag coefficient and Nusselt number of spheroids in SCW to depict the influence of the variable physical properties of SCW and incident angle of spheroids. However, these studies mentioned above all assume that the particles are ideal smooth geometries while real particles always tend to have rough surface during grinding and thermochemical conversion processing, which will deeply affect SCW-particle two-phase flow and heat transfer characteristics.
In solid-gas systems, surface roughness is an important parameter. Most work focused on studying the effect of roughness on momentum characteristic of flow past particles, especially in turbulent flow regime. A typical case is that the dimple surface on golf ball will greatly reduce the drag coefficient [18], [19]. The surface roughness will cause earlier boundary transition from laminar to turbulent at relatively low Reynolds number (Re) and the decrease of drag coefficient [20], [21]. While in trans-critical range, the higher roughness will produce larger drag coefficient [22]. Zhou et al. [23] investigated flow past a cylinder with fully dimpled and half dimpled rough surface. They revealed that the dimpled surface would reduce the mean drag and the effect of different orientation of the rough surface on the drag was significant. However, in laminar region, surface roughness has little effect on the flow characteristics such as pressure penalties and drag coefficient [24], [25].
Surface roughness also has a significant effect on the heat transfer characteristics both in laminar and turbulent region. Joss and Aufdermaur [26] found that the heat transfer of particle was enhanced as surface roughness increased. And the effect of roughness on liquid is greater than that in air at relatively low Re. Garcia et al. [27] compared the influence of different artificial roughness such as dimples, corrugated tubes and wire coils on the pressure drop and heat transfer. They concluded the recommended types of tubes used at different Re from laminar to turbulent to attain better heat transfer coefficient levels and lower pressure drop. Dierich and Nikrityuk [25] simulated flow past rough cylindrical particle for 10 Re 200. They found that Nusselt number decreased rapidly with an increase of roughness and built a relationship between the efficiency factor and surface enlargement coefficient.
Studies mentioned above all indicate that surface roughness plays an important part in gas–solid two phase flow and heat transfer characteristics. But the current SCW-particle models all ignore this phenomenon. In this paper, fully resolved numerical simulations of SCW flow past single rough sphere particle under laminar region were carried out to investigate the effect of surface roughness. Open source CFD software OpenFOAM is used for simulation.
Section snippets
Numerical method
In this paper, we consider the SCW flow past single rough sphere particle in the laminar region corresponding to the Re range from 10 to 200. Gravity effect is neglected. The governing equations can be expressed as: where is fluid density, is fluid velocity vector, is the pressure, is viscous stress tensor, is the special enthalpy, is the mechanical energy and is heat flux vector. The definition of , and is given as follows:
Results and discussion
Fig. 5 shows the properties of SCW under different temperature at P 23 MPa. From the figure we can see that the properties of SCW are very sensitive to temperature, especially near the critical point. To study the effects of surface roughness of particles in SCW and variable fluid properties. The temperature of inlet and surface of particle are set as 647 K, 657 K.
Conclusion
In this paper, fully resolved numerical simulation of SCW flow past single rough sphere particle in the range of 10 Re 200 was performed. The roughness model is produced by setting periodically distributed dimples on particle. And the effect of roughness on flow and heat transfer characteristics of particles are studied. Based on the simulation results, the important results are summarized as follows:
- 1.
As roughness increases, the separation bubbles appear in the dimple enhance the flow
Acknowledgments
This work is supported by the Basic Science Center Program for Ordered Energy Conversion of the National Natural Science Foundation of China (No. 51888103) and the National Natural Science Foundation of China (51922086).
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