Heat transfer and pressure drop in annuli with approximately uniform internal wall temperatures in the transitional flow regime

Abstract

Highlights• Experimental investigation of the transitional flow regime in an annulus for water.• Heated and cooled inner wall uniform wall temperature conditions are studied.• Nusselt number, friction factor and critical Reynolds number results are presented.• Annular dimensions directly impact resultsa new geometric parameter is proposed.• Nusselt number and friction factor correlations are developed. AbstractAn experimental study was conducted to determine the lower and upper Reynolds number limits of the transitional flow regime, and the characteristics of the heat transfer coefficients and friction factors for annular passages with different hydraulic diameters and diameter ratios in the transitional flow regime. Water was used in this investigation during isothermal, heating and cooling cases. Four horizontal concentric counter-flow tube-in-tube heat exchangers with conventional inlet geometries were considered to obtain the required data.The flow was both hydrodynamic and thermally developing, and the transitional flow was composed of mixed and forced convection types. The wall temperature on the inner surface of the annular passages was approximately uniform, while the outer surface was isothermal.Average Nusselt numbers were obtained for both the heating and cooling cases, while friction 1 Corresponding author: jaco.dirker@up.ac.za, Mechanical and Aeronautical Department, University of Pretoria, Private Bag X20, Hatfield 0028, RSA. 2 factors were obtained for heating, cooling and isothermal adiabatic conditions. Isothermal adiabatic condition was considered for reference purposes. The geometric size of the annular passage and direction of the heat flux (heating and cooling cases of annular fluid) had a significant influence on the heat transfer coefficients, friction factors and Reynolds number span of the transitional flow regime. The annular geometric parameters that represent the geometric size of the annular passage were proposed and found to describe the heat transfer coefficient and friction factors well. Subsequently, correlations for predicting the Nusselt numbers and friction factors in the transitional flow regime were developed.

Inlet tube spacing and protrusion inlet effects on multiple circular tubes in the laminar, transitional and turbulent flow regimes

“…Ndenguma et al [30,31] The literature review showed that previous work can be divided into two categories. The first category is literature on an application and/or system level where inlet effects on multiple tubes (much more than three tubes) were investigated [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15], and the research methodologies followed were not on a very fundamental level.…”

“…It was found that the degree of temperature uniformity on the inner surface of the annular passage had an influence on the transitional Reynolds number range, the heat transfer coefficients, as well as the friction factors. When the influence of the annular dimensions was investigated, an annular geometric parameter (λ = aL/D h ) that takesinto consideration both the annular diameter ratio and the hydraulic diameter, was proposed[31]. It was found that as the annular geometric parameter increased, the width of the transitional flow regime, as well as the magnitude of the Nusselt numbers and friction factors, decreased.…”

Thermal and electrical performance analysis of induction heating based‐thermochemical reactor for heat storage integration into power systems

“…where G m is the mutual irradiation coming from other boundaries in the system, G amb the ambient radiation source, F amb an ambient view factor, T amb the assumed far-away temperature in the directions included F amb , G ext the irradiation from external radiation sources equal to zero for this study, n is the refractive index, and σ the -Stefan-Boltzmann constant equal to 5.67 Â 10 À8 W/(m 2 ÁK 4 ). By combining Equations ( 24), (25), and (26), the radiosity at the diffuse surface is given as follows.…”

“…13 As for the reactor, its design and thermal performance are the relevant factors that significantly impact the thermochemical energy storage efficiency. [24][25][26][27]29 This study investigates the thermal and electrical performance analysis of induction thermochemical reactor for high-temperature heat storage. The induced heat distribution in the heat generator and heat storage medium is analyzed using the induction heating model coupled with heat transfer and laminar flow.…”

“…However, there are several factors identified in the literature affecting the efficiency of induction heating, such as the frequency and intensity of induced current; physical characteristics, dimensions, and shape of the workpiece; design configurations of the inductor; desirable temperature range, heating time, proper selection of power source, and induction heating's control system 13 . As for the reactor, its design and thermal performance are the relevant factors that significantly impact the thermochemical energy storage efficiency 24‐27,29 …”