Elsevier

Acta Astronautica

Volume 185, August 2021, Pages 89-101
Acta Astronautica

Dynamic flow instabilities of hydrocarbon fuel in a horizontal heating tube

https://doi.org/10.1016/j.actaastro.2021.04.016Get rights and content

Highlights

  • Three main types of dynamic flow instabilities are discovered during the test.

  • The decisive influence on dynamic flow instabilities is revealed.

  • Effects of experimental parameters on dynamic flow instabilities are discussed.

Abstract

To investigate dynamic flow instabilities of hydrocarbon fuel in thermal protection structure of the scramjet, experiment was conducted at the pressure of 1.5–3.0 MPa, heat flux of 190–250 kW⋅m−2, and the charge pressure at the surge tank of 0.5–1.0 MPa. Three main types of dynamic flow instabilities, pressure drop oscillation, density wave oscillation, and thermal oscillation took place during some experimental conditions. The influence of hydraulic characteristics of the test loop on dynamic flow instabilities was discussed. It was found that dynamic flow instabilities were mainly affected by the hydraulic characteristics of the test loop rather than the heating section. Pressure drop oscillation with large fluctuation amplitude and long period occurred in the negative slope region of the pressure drop curve. Density wave oscillation appeared in the positive slope region with low flow rate. The fluctuation amplitude and period of density wave oscillation were much smaller than those of pressure drop oscillation. Dynamic flow instabilities were more intense at lower pressure and higher heat flux. The oscillation region was independent of the compressible volume. However, a large compressible volume could promote the fluctuation amplitude of both inlet mass flow rate and outlet fuel temperature.

Introduction

The active regenerative cooling technology [1] has been viewed as the most effective thermal protection method for scramjet. In this technology, hydrocarbon fuel serves as both propellant and coolant. Before the fuel flows into the combustor, the fuel flows through the parallel multichannel in the combustion chamber wall and absorbs the aerodynamic heat and combustion heat. Usually, the fuel works at supercritical pressure. However, the pressure of the fuel decreases along the multichannel owing to the frictional and local resistance. The pressure of the fuel may reduce to subcritical pressure in some cases. At constant heat flux, the hydraulic characteristics curve (dP vs. m) is not a monotone curve, but a cubic curve for water and a quintic curve for hydrocarbon fuel [2]. If there is any compressible volume located on the upstream of the cooling channels, the cooling system will be subject to dynamic flow instabilities. Pressure drop oscillation (PDO) and density wave oscillation (DWO) are two common dynamic flow instabilities in heat-exchange equipment [3]. Usually, PDO occurs in the negative slope region of the hydraulic curve, and DWO appears in the low flow rate positive slope region. During dynamic flow instabilities, the mass flow rate, pressure, and fluid temperature fluctuate periodically, which will induce thermal fatigue of the cooling structure and combustion instability.

Since the flow instability is one of the serious issues in many industrial facilities, such as boilers and nuclear reactors, flow instabilities of water in natural circulation loop [[4], [5], [6], [7]] and force convection heating tube [[8], [9], [10], [11], [12]]have attracted much attention in recent years. Li [4] compared the stable boundaries of PDO in a natural circulation loop acquired by the experiment and RELAP5. It was found that these two boundary values were very close, but the RELAP5 result was less conservation than the experimental result. Lu [8] and Grzybowski [9] analyzed the evolution of two-phase flow instability combined with the transition of flow pattern. Both of them discovered that the onset of flow instability accompanying the flow pattern transferring into the annular flow. Al-Yahia et al. [10] discussed the effect of heat flux distribution on flow instability in a narrow rectangular channel. According to their result, the onset of inlet pressure fluctuation in the non-uniform heated test section was earlier than that in uniform heated section owing to high local heat flux. Two frequencies oscillation of supercritical water in parallel channels has been reported by Liang et al. [11]. They discovered that the low-frequency oscillation could be stabilized by increasing pressure, inlet pressure drop coefficient, or mass flux. However, the high-frequency oscillation was independent of pressure and inlet pressure drop coefficient. Lucas and Issam [13] summarized recent investigations on flow instabilities in macro- and micro-channel. They suggested that the influence of size and position of compressible volume on PDO required additional investigation.

The flow instabilities of R134a in a horizontal heating tube have been experimentally investigated by Dorao et al. [[14], [15], [16], [17]]. The heat transfer characteristics of R134a during DWO, the period of DWO, and the influences of heat flux distribution on PDO were thoroughly discussed in their publications. However, the experiments were conducted at relatively low heat flux and low fluid temperature, different from the application condition of active regenerative cooling technology.

Except for experimental investigation, many theoretical models have been developed for simulating DWO and PDO. Two different methods can be adopted to solve these models, namely the time-domain method [[18], [19], [20]] and the frequency-domain method [[21], [22], [23]]. Usually, the computation requirement of the time-domain method is heavier than that of the frequency-domain method. The stable boundary can be achieved by the frequency-domain method, but it is only suitable for analyzing DWO.

These publications listed above are concerned with flow instabilities of water or refrigerant. Only a few publications can be available on dynamic flow instabilities [[24], [25], [26]] and Ledinegg [2,27,28] instability of hydrocarbon fuel. Although some literatures [[29], [30], [31]] discussed thermo-acoustic instability of hydrocarbon, dynamic flow instabilities were essentially different form thermo-acoustic instability. The mass flow rate remains stable during thermo-acoustic instability. However, the periodical fluctuation in mass flow rate is a distinct character for dynamic flow instabilities.

Zhou [25] experimentally studied flow instability of n-decane in a mini tube with an accumulator fixed at the inlet of the tube. Additionally, a zero-dimension homogeneous model and small deviation linearization theory were applied to explore the mechanism of instability. It was found that the remarkable decrease in fuel density near the pseudo-critical temperature and cracking temperature was the main reason for flow instability.

Dynamic flow instabilities of supercritical kerosene in parallel multichannel were reported by Wang et al. [26]. A low-frequency oscillation in pressure and temperature was observed in their research. They considered this oscillation as DWO because the oscillation period was closed to the time of the fuel flow through the channel and was independent of channel configurations. However, the influences of experimental parameters on the period and fluctuation amplitude of DWO had not been clarified. Yan et al. [24] experimentally investigated flow instabilities of n-decane in a vertical tube. With an increase in heat flux, the flow state was divided into 7 different stability stages. The flow instabilities were restrained at higher pressure, inlet mass flow rate or fluid temperature or downward flow. The transition from laminar to turbulence and dramatic variations of thermal properties were the reasons for two different kinds of flow instabilities, while PDO was not involved in their research. As the hydraulic characteristics curve was crucially important for analyzing dynamic instabilities, Yang et al. [27,28] discussed the influences of pressure, heat flux, and inlet temperature on hydrodynamic characteristics of cyclohexane. The minimum point of the hydrodynamic curve was viewed as the onset point of flow instability, and a dimensionless correlation for predicting onset point was developed based on the π theorem. With an increase in pressure or inlet fluid temperature or a decreased in heat flux, the negative slope of the pressure drop curve become flatter and the stability of hydrodynamic characteristics was improved. However, the pressure drops of the preheating section and the condenser were ignored. Almost no investigators paid attention to the hydraulic characteristics of the condenser and other tubes in the test loop.

In conclusion, only a few publications discussed about dynamic flow instabilities of hydrocarbon fuel. It has reached a consensus that increase pressure could be beneficial to the flow stability. However, the influence of compressible volume on dynamic flow instabilities of hydrocarbon fuel has not been reported.

In this paper, dynamic flow instabilities of hydrocarbon fuel RP-3 in a horizontal heating tube were experimental investigated at different pressure, heat flux, and compressible volume. The basic characteristics of dynamic flow instabilities and variations of measured parameters during flow instabilities were revealed. Much emphasis was put on the hydraulic characteristics of the test loop. The influences of experimental parameters on fluctuation amplitudes of inlet mass flow rate and outlet fuel temperature were discussed.

Section snippets

Experimental apparatus

The experimental investigation was conducted at the test loop as shown in Fig. 1. China aviation kerosene RP-3 with the critical pressure of 2.33 MPa was used as the working medium. The fuel from the fuel tank flowed into a constant flow pump of which the output flow rate was independent of the working pressure. A surge tank was connected to the outlet of the pump to simulate the upstream compressible volume in the actual fuel supply system. There was an airbag with a volume of 0.6 L in the

Basic characteristics of dynamic flow instabilities of hydrocarbon fuel

At constant heat flux, the pressure drop exhibited as a multi-valued curve with a decrease in mass flow rate. When the system runs on the negative slope region of the pressure drop curve, PDO accompanying large amplitude and long period fluctuations in pressure and mass flow rate would occur. At the pressure of 1.5 MPa, heat flux of 250 kW⋅m−2, variations of the pressure drop of the test loop and outlet fuel temperature with a decrease in the output flow rate of the pump are plotted in Fig. 3.

Conclusions

Dynamic flow instabilities of hydrocarbon fuel in a horizontal heating tube were experimentally investigated in this paper. Basic characteristics of dynamic flow instabilities were introduced, and the influences of pressure, heat flux, and compressible volume on dynamic flow instabilities were discussed. The main conclusions are as followed.

PDO usually occurred in the negative slope region of the pressure drop curve. During PDO instabilities, inlet mass flow rate, system pressure, pressure

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.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51776167) and the National Science and Technology Major Project (2017-III-0005-0029).

References (36)

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