Effect of Protrusion in the Enhancement of Regression Rate in Hybrid Rocket

Abstract

It has been reported in literature that the use of protrusion in the combustion chamber of a hybrid rocket motor enhances the regression rate. This study reports careful experiments conducted to show that the improvement in burn rate with protrusions is only up to a certain fraction of the overall burn time. From the results obtained, it is seen that an X/L of 0.5 is the best location for a graphite protrusion. The protrusions are also shown to increase the combustion efficiency by as much as 45 % when placed at an X/L of 0.5. This, more than the improvement in the regression rate is very useful, especially for small scale motors whose combustion efficiencies are otherwise very low. Nomenclature Al= aluminum AP = ammonium perchlorate A t = throat area, m 2 C p = specific heat of propellant, kJ/kg-K C * exp = experimental characteristic velocity,m/s C * theo = theoritical characteristic velocity, m/s d i = intial port diameter, m d f = final port diameter, m G ox = oxidizer mass flux, g/cm 2 s I sp = specific impulse, N-s/Kg L g , L = length of fuel grain, m L = characteristic length, m m f = mass of fuel burnt, gṁ f = mass flow rate of fuel, g/ṡ m ox = mass flow rate of oxidizer, g/ṡ m tot = total mass flow rate, g/s n = flux exponent O/F = oxidizer to fuel ratio P c = average chamber pressure, bar P E = polyethylene t b = burn time, s X = axial distance of protrusion, m ρ f , ρ p = density of fuel, kg/m 3 η c * = combustion efficiency λ p = thermal conductivity of propellant, W/m-K τ p = thermal response time, s

A numerical investigation of oxidizer mixed hybrid rocket motors

“…The numerical model in the present study is explained in detail in [3]. Hence the model is only briefly discussed here.…”

“…The combustion chamber in a single port hybrid rocket motor is assumed to be axially symmetric. The governing equations considered are expressed in 2D axi-symmetric coordinates [3].…”

“…Nevertheless, thermodynamic effect of multi species can be readily incorporated [12]. For simulations involving AP mixed solid grains, the single step gas phase chemistry used in [3] is insufficient. Ramakrishna et al [13] carried out a numerical study in composite AP based sandwich propellant.…”

“…Typical regression rate enhancement techniques involve heat transfer improvement at the fuel surface by means of perturbed flow [1][2][3], improved injector design [4][5][6][7] or energetic additives to the solid fuel [8][9][10][11]. Disturbances in the flow field are typically caused by obstructions like diaphragms that form recirculation zones downstream of them.…”

Measurement of regression rate in hybrid rocket using combustion chamber pressure

“…The average value of combustion efficiency is obtained using Eqs. (10) and (11) [18,37]. It is seen that these are also fairly repeatable:…”