Heat transfer deterioration in upward and downward pipe flows of supercritical n-decane for actively regenerative cooling

https://doi.org/10.1016/j.ijthermalsci.2021.107066Get rights and content

Highlights

  • The inertial force and the viscous force play a major role in the upward and downward flow, respectively.

  • A large velocity gradient in the buffer layer and the non-monotonicity of velocity contribute to the HTD.

  • A thermal barrier is generated under the large heat diffusion near the heated wall and thus causes the HTD.

Abstract

In this paper, we consider the flow and heat transfer behaviour of turbulent upward and downward flows of supercritical n-decane, in order to reveal the features of heat transfer deterioration (HTD) that would be expected in relevant active regenerative cooling systems for scramjet engines. Specific focus is placed on key velocity-field features that appear in these flows. Following the validation of six turbulence models, the SST k-ω and RNG k-ϵ models are found to be suitable for simulating the upward and downward flow cases, respectively. “M” type velocity profiles (a non-monotonicity of the velocity along the radial direction) are observed, which arise due to a spatially-varying interplay between the inertial and viscous forces in the flow domain, while larger velocity gradients in the buffer layer are also observed that contribute to the phenomenon of HTD. Furthermore, it is found that the secondary flows as well as the different mass fluxes that arise due to the velocity increase from the wall to the flow core zone (i.e., the influencing range and intensity of cross-sectional kinetic energy), respectively, are observed in the HTD development region, as well as the HTD peak area and degradation regions. A zone of higher thermal diffusion appears in the near-wall region, which acts as a thermal barrier and contributes to HTD.

Introduction

Hypersonic vehicles are considered as a new aerial combat platform due to perfect stealth, a high Mach number flight and a wide range of strikes. The scramjet, which uses a hydrocarbon fuel as a propellant, is widely regarded as the core component of hypersonic vehicles [1]. The heat load of the combustor increases sharply due to the action of supersonic combustion and aerodynamic heating. The temperature and heat flux at a Mach number of 8 are 4000 K, 10–20 MW/m2, respectively, and the temperature exceeds the limits of existing materials [2]. An active regenerative cooling system can perfectly solve this problem [[3], [4], [5]] and the hydrocarbon fuel, e.g., RP-3 is used as a refrigerant in cooling channels. Generally, the critical pressure of the hydrocarbon fuel, e.g., RP-3, is 2–3 MPa and these always work at supercritical condition due to the operating pressures of 3.0–7.0 MPa of the combustor [6,7]. The thermophysical properties of supercritical hydrocarbon fuels change drastically and abnormal heat transfer phenomena can be observed such as a heat transfer deterioration (HTD) [[8], [9], [10], [11]]. Thus a large number of research works has paid attention to the convection heat transfer characteristics of supercritical hydrocarbon fuels. Because the components of hydrocarbon fuels are numerous and it is not realistic to consider them all in numerical simulations. Thus, surrogate models of hydrocarbon fuels are receiving much more attention [[12], [13], [14]]. In this work, supercritical n-decane is considered to be the refrigerant in the cooling channels of the active regenerative cooling system.

In addition to the studies on supercritical hydrocarbon fuels, supercritical water and carbon dioxide (CO2) in pipes with different types and orientations, i.e., vertical [15,16], horizontal [17,18] and helically coiled [19,20] pipes have also attracted attention, in particular for supercritical CO2, which has been poven to be a promising working fluid to be used in heat-to-power systems [[21], [22], [23]]. An abnormal heat transfer phenomenon can be found collectively in the thermal performance of these supercritical fluids. Fu et al. [24] and Jiang et al. [25] experimentally studied the different influence factors on the heat transfer of supercritical RP-3 and carbon dioxide such as system pressure, heat flux, mass flow rate, flow direction, etc. They reported that the sharp variation of the thermophysical properties plays an important role in the deteriorated heat transfer zone. Furthermore, Pioro et al. [26] deemed that the occurrence of the HTD can be accurately evaluated by one correlation. Li et al. [27] used the Shear-Stress Transport (SST) k-ω model to investigate the flow and heat transfer of supercritical water flowing in internally ribbed tubes. They reported that ribbed geometries hardly work but mixed ones play an important role by suppressing the buoyance force. Jackson [28] studied the flow and heat transfer in the presence of a buoyance force in a vertical tube and found that the forced convection turns into free convection with an increase of the buoyancy force. He et al. [29] experimentally researched the heat transfer of supercritical R245fa flowing vertically upward in a circular tube and it was observed that the phenomenon of heat transfer deterioration appeared at moderate heat and mass fluxes. Liao and Zhao [30] numerically explored laminar convection of CO2 in a vertical mini/micro tube and it was revealed that the buoyancy effect plays an important role in a small tube even for high Reynolds numbers. Bovard et al. [31] numerically investigated the heat transfer of supercritical CO2 and water and it was demonstrated that the increase of mass flux and operating pressure can decrease the effect of thermal-induced acceleration and buoyance force. These conclusions are consistent with Ref. [32]. Pucciarelli et al. [33] used LES (Large Eddy Simulation) to analyse the coupling effect of fluid heat transfer and wall heat conduction. It was suggested that the function of wall heat conduction should be considered explicitly. Tao et al. [34] proved that the buoyancy, density fluctuation and variation profoundly influence the accuracy of turbulence models. By modifying the LS (Launder-Sharma) model, the precision was improved by 41% compared to the experimental data. Kline et al. [35] experimentally explored the onset of HTD for CO2 flowing upward in electrically heated vertical tubes and the minimum heat flux was given. Jaromin and Anglart [36] investigated heat transfer of supercritical water in a vertical tube at deteriorated conditions and they also found that the wall temperature is greatly affected by the Prandtl number. Sun et al. [37] proposed a prediction model based on artificial neural network (ANN) to predict the heat transfer characteristics under various conditions and the results revealed that the model was capable of capturing the underlying heat transfer mechanism compared to traditional heat transfer correlations.

Through the above-mentioned literature survey, the parametrical variation, buoyancy effect, heat transfer correlation, heat transfer of enhanced structures, prediction models of heat transfer deterioration, modifications of the turbulence models on the abnormal heat transfer of supercritical fluids were widely studied. However, the mechanism of heat transfer deterioration is still unclear and this point was confirmed in Ref. [38]. In this context, the motivation here is to study the fundamental mechanism of heat transfer deterioration. It is well known that the effect of the flow field on the temperature field is very strong but a more detailed analysis of the flow field was not mentioned in the open literature. In this paper, the information of flow behaviour is used to illustrate the heat transfer characteristics, since the velocity distribution and flow structure demonstrate the heat transfer intensity in some certain areas, and the velocity fields and velocity gradients are used to indicate the global velocity structures and velocity trends, respectively. The velocity vector is applied to show the flow direction and the effect of secondary flow on the abnormal heat transfer, particularly for heat transfer deterioration, is also explained.

Section snippets

Active regenerative cooling

As described above, the active regenerative cooling technique of interest to the present work is regarded as a particularly effective approach for absorbing the combustor-generated heat as shown in Fig. 1. The hydrocarbon fuel firstly flows into channels and the heat in the combustor wall is taken up by the hydrocarbon fuel. The heated fuel inters the combustion chamber to participate in the combustion and generate thrust. In addition, heated fuel is used as a heat source in the power

Thermophysical properties of n-decane

Before the numerical calculations are conducted, the physical properties of the supercritical fuel should be determined and these data can be calculated by NIST (National Institute of Standards and Technology) [48]. The variations of the thermophysical properties of n-decane with the change of fluid temperature can be found in Refs. [49,50] and are also plotted in Fig. 3. Herein, the function of piecewise-linear in the commercial software Fluent was applied and 30 points were embedded. It

Heat transfer partitions

Fig. 6 shows how the inner wall temperature is changing along the flow direction for the upward-flowing and downward-flowing cases. There is a large difference between the two cases in terms of the inner wall temperature and this means that the heat transfer behaviour is inconsistent for different flow directions. For the upward-flowing case, an HTD (a typical form of heat transfer) can be observed. Generally, the heat transfer can be separated in the following regions: inlet region, normal

Conclusions

In order to cool effectively the combustion chambers of scramjets, turbulent flows of supercritical n-decane are used to absorb the heat in cooling channels. Abnormal flow and heat transfer behaviour is always observed in such flows due to the dramatic variation of thermophysical properties of supercritical fuels, particularly near the pseudo-critical point. To understand the physical mechanisms of unconventional flow and heat transfer, the SST k-ω and RNG k-ϵ turbulence models were used to

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was sponsored by the National Natural Science Foundation of China (51676163), the National 111 Project (Grant No. B18041), the Fundamental Research Funds of Shenzhen City (JCYJ20170306155153048), and the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (Grant No. CX202029), and was financially supported by the China Scholarship Council (CSC) who gave Mr. Y. Li the opportunity to perform part of his PhD studies at Lund University. The work was also

References (54)

  • T.H. Kim et al.

    Experimental investigation on validity of buoyancy parameters to heat transfer of CO2 at supercritical pressures in a horizontal tube

    Exp. Therm. Fluid Sci.

    (2018)
  • K.Z. Wang et al.

    Experimental and numerical investigation on heat transfer characteristics of supercritical CO2 in the cooled helically coiled tube

    Int. J. Heat Mass Tran.

    (2017)
  • W. Zhang et al.

    Mixed convective heat transfer of CO2 at supercritical pressures flowing upward through a vertical helically coiled tube

    Appl. Therm. Eng.

    (2015)
  • J. Song et al.

    Parametric optimisation of a combined supercritical CO2 (S-CO2) cycle and organic Rankine cycle (ORC) system for internal combustion engine (ICE) waste-heat recovery

    Energy Convers. Manag.

    (2020)
  • J. Song et al.

    Combined supercritical CO2 (SCO2) cycle and organic Rankine cycle (ORC) system for hybrid solar and geothermal power generation: Thermoeconomic assessment of various configurations

    Renew. Energy

    (2021)
  • Y.C. Fu et al.

    Experimental investigation on convective heat transfer of supercritical RP-3 in vertical miniature tubes with various diameters

    Int. J. Heat Mass Tran.

    (2017)
  • P.X. Jiang et al.

    Experimental and numerical investigation of convection heat transfer of CO2 at supercritical pressures in a vertical tube at low Reynolds numbers

    Int. J. Therm. Sci.

    (2008)
  • I.L. Pioro et al.

    Heat transfer to supercritical fluids flowing in channels—empirical correlations (survey)

    Nucl. Eng. Des.

    (2004)
  • Z.H. Li et al.

    Effects of rib geometries and property variations on heat transfer to supercritical water in internally ribbed tubes

    Appl. Therm. Eng.

    (2015)
  • J.D. Jackson

    Models of heat transfer to fluids at supercritical pressure with influences of buoyancy and acceleration

    Appl. Therm. Eng.

    (2017)
  • J.C. He et al.

    Experimental investigation of heat transfer to supercritical R245fa flowing vertically upward in a circular tube

    Int. J. Heat Mass Tran.

    (2018)
  • S. Bovard et al.

    Numerical investigation of heat transfer in supercritical CO2 and water turbulent flow in circular tubes

    J. Supercrit. Fluids

    (2017)
  • A. Pucciarelli et al.

    On the effect of conjugate heat transfer on turbulence in supercritical fluids: results from a LES application

    Ann. Nucl. Energy

    (2018)
  • Z. Tao et al.

    Correction of low-Reynolds number turbulence model to hydrocarbon fuel at supercritical pressure

    Aero. Sci. Technol.

    (2018)
  • N. Kline et al.

    Onset of heat transfer deterioration in vertical pipe flows of CO2 at supercritical pressures

    Int. J. Heat Mass Tran.

    (2018)
  • M. Jaromin et al.

    A numerical study of heat transfer to supercritical water flowing upward in vertical tubes under normal and deteriorated conditions

    Nucl. Eng. Des.

    (2013)
  • F. Sun et al.

    Thermal characteristics of in-tube upward supercritical CO2 flows and a new heat transfer prediction model based on artificial neural networks (ANN)

    Appl. Therm. Eng.

    (2021)
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