Experimental investigation on natural convective heat loss of cylindrical cavity with/without a quartz window

https://doi.org/10.1016/j.ijthermalsci.2019.106220Get rights and content

Abstract

This paper presents an experimental setup for the study of the natural convective heat loss characteristics of a cylindrical cavity with/without a quartz window. The thermal performance such as wall temperature and heat loss of two types of cavity receivers are studied by changing the tilt angles and heated states of the walls, then the effect of quartz window on the heat loss of the cavity is highlighted. The average temperature of cavity with a quartz window is respectively 68.61 °C, 79.36 °C, 48.22 °C higher than same cases without a quartz window (corresponding tilt angle is −90°, −30° and 45°). The temperature difference is within 10 °C for quartz window cavity and 40 °C for no quartz window cavity when q = 700 W/m2 and all surfaces are heated. More than 70% cases occur that the natural convective heat loss of no quartz window cavity is greater than that of the quartz window cavity. The experimental results show that the quartz window can significantly increase the operating temperature of the cavity and weaken the influence of tilt angle. The quartz window plays an important role in the balance of cavity temperature and reduction of natural convective heat loss. The empirical correlations of predicting the radiant and natural convective Nusselt numbers of quartz window cavity considering tilt angle and heat flux have also been obtained.

Introduction

The rapid development of the economy and improvement of human living standard accelerate the energy consumption, while the utilization of traditional energy is limited by the environment and the total amount of resources. Solar energy has been widely concerned for its safe, environmentally friendly, and renewable characteristics. Solar thermal power generation can utilize a full range of sunlight, and it can also generate power day and night continuously through heat storage, which is receiving more and more attention. The cavity receiver is an important photo-thermal conversion component in the solar thermal power generation system, and its performance directly affects the efficiency of the whole system. Therefore, it is of great significance to study the thermal performance of the solar receiver.

The presence or absence of a quartz window (quartz glass cover) at the opening of cavity significantly affects the mechanism of convection heat transfer [1]. As for the downward-facing cavity without a quartz window, the cold air invades the cavity beginning with tilt angle ϕ = 0°, the hot air escapes and takes thermal energy away. The volume of high-temperature air is less than half of the cavity volume, and the hot air tends to be stable until the cavity is straight downward [2]. For a cavity with the quartz window, the interior of the cavity is separated from surroundings, the quartz window is the only way to proceed convection heat transfer with the outside of the cavity. The selective coating on the surface of the quartz window can intercept the infrared radiation inside the cavity wall, greatly reduce natural convective and radiant heat loss, improve the operating temperature of the cavity [3,4]. The quartz window can also resist external impurities and reduce damage to system caused by the environment.

Many numerical and experimental investigations have been carried out on cavity receivers without any quartz window. On the contrary, the study on the cavity with the quartz window is comparatively rare. For no quartz window cavities, the early experiments focused on square cavities, and other geometrically shaped cavities were studied in later researches. Prakash et al. [5] carried out a numerical investigation on cubic cavity, hemispherical cavity and spherical cavity with the same heat transfer area. Jilte et al. [6] established three-dimensional models for the cavity of different shapes (conical, cylindrical, cone-cylindrical combination, dome-cylindrical combination, and cone-conical combination), and studied the influence of variables such as wind direction, tilt angle and aperture ratio on the thermal performance of cavity under constant wall temperature. It is found that an increase in the aperture ratio of the cavity may even increase the convective heat loss in some cases. Wu et al. [7] systematically summarized the research and progress in the field of cavity heat dissipation, including various cavity types involved in engineering applications, and pointed out their advantages and disadvantages and the further research directions. In addition to the volumetric cavity, there are a lot of studies on heat pipe and tubular cavity. Shewale et al. [8] and Karimi et al. [9] carried out experimental and numerical simulation studies on spherical and cylindrical tubular cavities respectively, the variables include mass flow rate of heat transfer fluid, solar radiation intensity, cavity diameter and height and so on. The numerical study of Du et al. [10] showed the temperature distribution of a single tube in solar tower receiver, the thermal stress was calculated in combination with relevant equations. Furthermore, the minimum heat flux formula for the damage of the tube wall was also fitted. Wan et al. [11] proposed a model that combines light transmission, light-heat conversion, and thermal stress fatigue to study the thermal stress of a concentrated solar cavity receiver. On the basis of this model, the thermal performance of two boiling panels with different membrane walls was studied. Pavlovic et al. [12] found that spiral receivers present more uniform distribution of heat flux than conical cavity receivers, while conical cavity receivers have higher thermal efficiency and exergy efficiency. Loni et al. [13] pointed out that the conical cavity taking thermal oil as working fluid has high thermal efficiency and good commercial application prospects. Wu et al. [14] introduced a heat pipe receiver suitable for medium-temperature and high-temperature systems and found that the convection heat transfer area increases obviously when the aperture ratio increases, so the natural convective heat loss of the cavity increases. The simulation results of Yang et al. [15] showed that the radiant and convective heat loss accounts for more than 90% of the total heat loss of the cavity, which is the research focus of improving thermal performance.

The experimental data of Loni et al. [16] on the hemispherical tubular cavity showed that the ratio of conductive heat loss to mixed convective heat loss is between 2% and 10%, that is, the heat loss of cavity is mainly affected by the internal flow. Therefore, it is recommended to cover the opening of the cavity with glass plate to reduce heat loss and improve the thermal efficiency. Different from traditional research results, the simulation of Reddy et al. [17] showed that the forced convective heat loss at low wind speed (less than 2.5 m/s) is lower than the natural convective heat loss. The forced convective heat loss of the improved cavity receiver of Yang et al. [18] is about 58% less than that of the natural convective heat loss under optimal conditions. Uzair et al. [19] believed that the interaction between wind and cavity alters the airflow velocity at the opening, which affects the convective heat loss. The forced convective heat loss is not necessarily higher than the natural convective heat loss. The temperature on the surface of the cavity wall is more uniform when the cavity is covered by quartz glass [20]. Compared with the cavity without a quartz window, the redistributing effect on the light of the plano-convexo quartz window also makes the heat flux distribution of the cavity more uniform [21]. At the same time, the selective coating on the surface of the quartz glass contributes greatly to reducing the heat loss of the cavity, especially the radiant heat loss [22]. Soo et al. [23] and Xiao et al. [24] studied tubular cavities with different shapes for high-temperature fluid, and found that quartz glass can reduce convective heat loss at the aperture, and the working fluid in the cavity with a quartz window can obtain more heat. Chang et al. [25] investigated the heat loss of hemispherical cavity, modeled the cavity with or without secondary trumpet reflector and quartz window. The heat loss of the cavity with secondary reflector and quartz window is minimal, wherein the quartz window contributes more to the reduction of heat loss than the secondary reflector. Sahoo et al. [26] studied the heat loss of a trapezoidal cavity used in a linear Fresnel system. Tsekouras et al. [27] investigated the temperature of the various components in the cavity receiver and found that the temperature of the glass cover is much smaller than that of the cavity receiver. Du et al. [28] simulated the heat transfer and pressure characteristics of a pressurized cavity receiver with a quartz window. The study showed that the focusing strategy has a distinct impact on the thermal stress of the quartz window. In terms of thermal efficiency, Soo et al. [23] concluded that the thermal efficiency of the cavity with a quartz window is about 17% higher than that of the cavity without a quartz window under certain conditions. However, Shuai et al. [21] thought that cavity efficiency is related to the reference temperature. The efficiency of the cavity without a quartz window varies greatly with temperature and the efficiency decreases when the temperature rises. The cavity efficiency of plano-convexo quartz window has no obvious difference with the change of temperature.

According to the above researches, many scholars have studied the influence factors such as the shape, tilt angle, aperture ratio and heat flux of the cavity without a quartz window. The quartz window has a positive impact on the reduction of the temperature gradient, the improvement of airflow pattern and temperature distribution [8,25]. However, there are few studies of the cavity with a quartz window, most of which are numerical simulations or based on the boundary conditions of constant wall temperature for tubular cavity receiver, and some researches such as the viewpoints about thermal efficiency of with/without a quartz window cavity in Refs. [21,23] are even inconsistent. To be specific, Shuai et al. [21] thinks the efficiencies of two cavities are related to the reference temperature, while Soo et al. [23] holds the idea that efficiency of the cavity with a quartz window is always higher than the no quartz window cavity. The tubular cavity requires a high-temperature selective coating to increase the absorption of the incident light, while the volumetric cavity does not require the coating except for the quartz window. Until now, there are few experimental researches on volumetric cavity receiver. Considering the advantages of the simple structure, the convenience of manufacture and relatively little heat loss of the cylindrical cavity, this experiment chooses cylindrical volumetric cavity as the research object, and a small aspect ratio is designed to meet the requirements of large input power [29] and high thermal efficiency [30] in practice.

In this paper, an experimental setup has been built to test the heat loss of the cylindrical cavity with or without a quartz window under no-wind condition, aiming to find out the role of the quartz window. By the analysis of temperature under different tilt angles and heat fluxes, the change rules of heat loss, natural convective and radiant Nusselt numbers are investigated. Based on those, the heat transfer mechanism of the quartz window cavity is explored; meanwhile, the empirical correlations of Nusselt numbers for quartz window cavity with consideration of heat flux, tile angle and other factors are also proposed.

Section snippets

Experimental apparatus

In the solar thermal power generation system, the main processes include the concentration of sunlight, the collection of heat, and the conversion of thermal energy into electric energy. This study aims to investigate the effect of the quartz window on the heat loss characteristics of the cavity receiver, so the concentration and reflection of sunlight and other processes are not included. The experiment was carried out indoors and the electric heating device attached on the cavity wall was

Experimental principles

Considering the good performance of the heating device and the thermal insulation, assume that the heat energy converted by the output power is completely absorbed by the cavity, so the power is equal to total heat loss of the cavity. The formula for the total heat loss is as followsQ=UsIs+UbIbWherein, Us, Ub represent the voltage applied to the side and bottom of the cavity respectively, V; Is, Ib represent the current applied to the side and bottom of the cavity respectively, A.

The conductive

Results and discussion

The thermal performance of the cavity is mainly characterized by temperature and heat loss. So the temperature and heat loss are analyzed and discussed separately.

Conclusions

The natural convective heat loss performance of the cylindrical cavity with a quartz window has been studied experimentally. The variation curves of cavity temperature and heat loss under different tilt angles and heated states of the walls are obtained, and the variation law is clarified. The thermal performance of the cavity without a quartz window is also studied for comparison purpose; hence, the role of the quartz window is further understood. The conclusions are as follows.

  • (1)

    The quartz

Acknowledgment

This work is funded by Project No. 2018CDXYDL0001 supported by the Fundamental Research Funds for the Central Universities.

Nomenclature

A1
area of the inner bottom surface of the cavity, m2
A2
total area of the inner surface, m2
Aap
area of the aperture, m2
b(Nu)
systematic standard uncertainty of Nu
b(Xi)
systematic standard uncertainty of Xi
b(XiYi)
correlation between standard uncertainty components caused by systematic influences of Xi and Xj
b(Y)
systematic standard uncertainty of Y
d1
outer diameter of the cavity, m
dc
inner diameter of the cavity, m
g
acceleration of gravity, 9.8 m/s2
Gr
Grashof number
H
inner height of the cavity, m
Ib
current

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