Numerical investigation of heat transfer characteristics between two particles and supercritical water

Highlights

• Heat transfer between two particles and supercritical water are studied.

• Changes in physical properties have no special effect on heat transfer trend.

• Heat transfer rate is affected by temperature gradient and physical properties.

Abstract

This work takes two particles as an example to study the effect of distance and angle between two particles on the average Nusselt number Nu, average heat transfer rate η_b, local Nusselt number Nu_φ, and local heat transfer rate η_aφ. It was found that the variation trend of Nu and Nu_φ in supercritical water is the same as that in constant property flow, thus indicating that the variation trend of heat transfer between particles and fluid in supercritical water is the same as in constant property flow. The change in physical properties of supercritical water does not have any special effect on the variation trend of heat transfer. The heat transfer is mainly affected by the overlap of temperature field and the overlap of flow field. However, the results of η_b and η_aφ indicate that the variation trend of heat transfer rate is affected by temperature gradient changes and physical property changes.

Introduction

Energy shortage and environmental pollution are twin problems that need urgent solutions. Using existing resources is particularly important for the development of new energy. Supercritical water (SCW) has gradually been widely used in the development of new energy. There are two aspects in the development of new energy at this stage: nuclear reactors with SCW as working fluid, and conversion of particles in SCW. Both these methods are related to the flow and heat transfer characteristics of SCW. In terms of nuclear energy, convective heat transfer between SCW and tube bundles has always been a particularly important research direction, and several studies have been done in this field. The conversion of particles in SCW has also received extensive attention in recent years, and the surface convective heat transfer characteristics of particles in SCW also constitute a hot topic of research. In the research on convective heat transfer of SCW flowing through the tube bundle, some early works such as Griem [1], Shitsman [2], Bishop et al. [3], and McAdams et al. [4] have developed some classical convective heat transfer correlations between SCW and tube bundle. In recent years, some studies, such as Mokry et al. [5], Gupta et al. [6], Cheng et al. [7], and Chen et al. [8], have also developed new correlations. Debrah et al. [9] used Waata's numerical data [10] to verify the aforementioned correlations. Gu et al. [11], [12], [13] used a 2 × 2 rod bundle to verify the aforementioned correlation by numerical simulation, and they found that Bishop is more in line with it. Supercritical water gasification technology is required for the conversion of particles in SCW [14], [15], [16], [17], and this technology has also received extensive attention in recent years. Supercritical water gasification technology is widely used, such as plastic degradation [18], coal gasification hydrogen production [19], [20], [21], and other areas. Regarding the research on the surface flow characteristics of particles in SCW, Wang et al. studied the effect of particle surface roughness on the drag coefficient [22]. Furthermore, due to the variable physical properties of SCW, Wang et al. studied the drag coefficients of two spherical particles in SCW [23]. Existing research shows that the temperature of particles in the process of SCW conversion is a major factor affecting gasification efficiency [19], [20]. Therefore, many works have also conducted research on this field. Wei studied the heat transfer coefficient on the surface of spherical particles in SCW, and finally developed a new correlation [24]. Wu et al. studied the effect of SCW properties on the surface heat transfer coefficient of spherical particles and fitted the corresponding correlations under different surface enthalpy values [25].

This study focuses on the transformation of particles in SCW. Summarizing the aforementioned studies, it was found that the research of heat transfer between SCW and particles is conducted on a single particle. Due to the drastic changes in physical properties of SCW [26] (see Fig. S1 of the Supplementary material), there are few studies on the heat transfer characteristics between particle clusters and SCW. Particle clusters are widespread in supercritical water fluidized beds (SCWFB); therefore, particle clusters are worth investigating. This study takes two particles as an example to analyze the heat transfer characteristics between the SCW and the two particles. Because SCW is characterized by high temperature and high pressure, the experiment is restrictive. This study adopts a numerical simulation method for research.