Effect of multi-location swirl injection on the performance of hybrid rocket motor

Highlights

• Use of multi-location swirl injection gives improved performance in hybrid rocket.

• 24 L/D motor did not gave improvement in performance with single swirl injector.

• Multi-location swirl in 24 L/D motor gave twice improvement in regression rate.

• Swirl injection with cavity at the mid gave twice improvement in regression rate.

• Swirl direction of multi-location injector had no influence on the performance.

Abstract

An attempt has been made in the present study to realize the effect of multi-location swirl injection on the improvement of regression rate and combustion efficiency of hybrid rocket by using two different length to diameter (L/D) ratio of the motor. It is 12 and 24. The fuel used for the study was Polyvinyl chloride (PVC)-Dibutyl Phthalate (DBP) in the ratio of 50:50. The gaseous oxygen was used as the oxidizer. The significance of this method is that a single swirl injection at the head end of the hybrid rocket is not sufficient to provide swirling flow till the nozzle end, if the length of the motor is sufficiently long and the regression rate decreases beyond certain length. In the present study, it has been observed that the 220 mm length is sufficient to get the improvement with single swirling injection. For the 24 L/D motor, the use of multi-location swirl injector motor gave twice the improvement in regression rate compare to the conventional hybrid rocket motor where showerhead injector has been used. Combustion efficiency obtained with multi-location swirl injection gave lower improvement with 24 L/D motor (from 70% to 76%), but the improvement was significant with 12 L/D motor with single swirl injector (from 50% to 74%). Improvement in regression rate as well as the efficiency was observed to be significant, if the mid injector was used as the cavity instead of supplying oxygen through it. The value of mass flux exponent (n) was observed to be 0.51 with multi-location swirl injection and the relatively uniform regression rate from head end to the nozzle end was observed.

Introduction

Hybrid rocket is a type of chemical rocket that has fuel in solid phase and oxidizer is either in liquid or gaseous phase. Due to these combinations of the propellant in hybrid rocket, it has various advantages compared to other chemical rockets such as safety, thrust controllability, low developmental cost, high reliability and less complexity than liquid rocket engines [[1], [2], [3], [4], [5], [6], [7], [8]]. These features of hybrid rocket are gaining more and more attention nowadays towards its usage in practical applications such as space tourism vehicle and as a means of transportation vehicle from one place to another [[9], [10], [11], [12], [13], [14], [15]]. But due to its certain drawbacks such as low regression, low combustion efficiency and non-uniform regression rate along the length of motor hinders its large-scale usage. In order to overcome these issues, various methods have been suggested by the researchers and it includes the use of energetic particles [[16], [17], [18], [19], [20], [21]], the use of oxidizer in limited quantities to the hybrid fuel [[21], [22], [23], [24], [25]] and controlling the flow of oxidizer in combustion chamber such that oxidizer mass flux can be enhanced near the fuel surface [[26], [27], [28], [29]]. Apart from these, the liquefying fuel such as wax has also been utilized to get the higher regression rate, but the drawback is that it has poor mechanical properties [30,31]. Although various attempts were made to improve the mechanical properties of these fuels by adding certain additives but the regression rate were observed to be reduced after improving its mechanical properties [31,32].

Among the various methods studied till date, the use of swirl injector was observed to give higher regression rate and combustion efficiency. The swirl injector creates an additional tangential velocity component besides the axial component of convention showerhead injectors in the hybrid rocket motor. With these augmented velocity in swirl injector, when oxidizer streams flow over the fuel surface, it effectively reduces the thickness of the boundary layer. When the boundary layer thickness is reduced, the flame is pushed closer to the fuel surface and it increases the heat feedback to the fuel grain which increases the regression rate of fuels. Besides this, swirl injector also create higher mass flux near the fuel surface with the increased residence time, effectively increases the regression rate and the efficiency of the hybrid rocket motor. Summers and Villarreal [33] had studied with swirl injector and used nitrous oxide as oxidizer and hydroxyl-terminated polybutadiene as the fuel. The swirl injectors used had different swirl angle and swirl flow numbers. It was reported that a 60-degree swirl flow injector gave as high as 1.63 times the regression rates obtained with zero-degree swirl flow injectors for the similar oxidizer mass flux conditions. Nearly uniform regression rate along the grain length for the highest swirl flow number was observed. They also reported that the oxidizer mass flux and swirl flow number are the leading variables in enhancing the regression rate of the hybrid rocket motor. Pucci [34] had utilized the nitrous oxide and polyethylene propellant combination to study the effect of swirl-injector on the flame holding combustion instability. They have reported that the 60⁰ swirl injectors produce stable combustion than radial injector. The 60⁰ swirl injector also showed a 182% rise in regression rate as compared to the axial and radial injectors. Lee et al. [35] had conducted studies to investigate the effect of both swirl injector along with helical grain on the enhancement of regression rate. The helical configuration was represented by the pitch number. Their results showed a higher regression rate with pitch 6 than with pitch 100 and it was observed for both either swirl or no swirl condition. Kumar and Kumar [36] had concluded that for a long motor L/D > 5, the swirling oxidizer flow created at the head end cannot maintain its swirling effect till the nozzle due to the shear stress acting at the fuel surface. These shear stresses at the fuel surface will diminish its swirling velocity and after certain distance flow becomes axial losing its swirling characteristics. As a result, head end region where there is swirling flow regresses faster than near the nozzle region due to the flow being axial. This phenomenon is called swirl decaying. Saburo et al. [37] had also confirmed through experiments that the enhancement in regression rate is severely localized near the injector side causing an unbalance in port diameter after combustion. Thus, to increase regression rate it is not appropriate to depend only on the headend swirling oxidizer flow unless a complimentary method has been utilized to reduce this unbalance in the fuel regression rate.

In order to address this problem concept of multi-section swirl injection, where oxidizer is injector into combustion chamber at multiple locations along the length of the motor has been adopted. A research group from Kyushu University from Japan [[38], [39], [40], [41], [42], [43], [44]] had utilized the multi-section swirl injection methods to get higher regression rate and the combustion efficiency. In this method, the oxidizer has been injected at various sections of the hybrid rocket motor. The oxidizer used for the study was oxygen while the fuels used were paraffin wax and the HDPE (High Density Polyethylene). The maximum length of fuel grain tested with this swirl injection method was 200 mm. They observed an improvement in the regression rate by around 3 to 10 times depending on the types of injection pattern combinations used. The maximum efficiency obtained was even higher than 100%. They also made an attempt to conduct the flight experiments by using this injection method. They reported that the flight was smooth and combustion was stable. Li et al. [45] had also conducted numerical studies with the use of multi-section swirl injection methods. The propellant combinations used were 98% hydrogen peroxide and polyethylene. The length of the fuel grain used by them was 200 mm. They reported that the average fuel regression rate increases by 8.37 times of conventional head end injector while efficiency improves by 95.73%. Apart from this, they also suggested that the increase in the number of injection section would increase the regression rate as well as the combustion efficiency. The regression rate and the combustion efficiency decrease with the reduction in the number of injection port in each injection section.

It is noticed from the studies conducted with the multi-section swirl injection method that the research is limited to a maximum length of around 200 mm and also within a very low oxidizer mass flux. In the present study, attempt has been made to study the combustion performance of hybrid rocket by utilizing the multi-section swirl injection methods using two different L/D ratio motor. The performance parameters used for the studies are regression rate, combustion efficiency and the sliver loss. It is further compared with the conventional shower head injectors. The effect of flow variation on the multi-section injection was also studied.