R. J. Stalker

Experiments on supersonic combustion ramjet propulsion in a shock tunnel

Measurements have been made of the propulsive effect of supersonic combustion ramjets incorporated into a simple axisymmetric model in a free piston shock tunnel. The nominal Mach number was 6, and the stagnation enthalpy varied from 2.8 to 8.5 MJ kg-1. A mixture of 13% silane and 87% hydrogen was used as fuel, and experiments were conducted at equivalence ratios up to approximately 0.8. The measurements involved the axial force on the model, and were made using a stress wave force balance, which is a recently developed technique for measuring forces in shock tunnels. A net thrust was experienced up to a stagnation enthalpy of 3.7 MJ kg-1, but as the stagnation enthalpy increased, an increasing net drag was recorded. Pitot and static pressure measurements showed that the combustion was supersonic. The results were found to compare satisfactorily with predictions based on established theoretical models, used with some simplifying approximations. The rapid reduction of net thrust with increasing stagnation enthalpy was seen to arise from increasing precombustion temperature, showing the need to control this variable if thrust performance was to be maintained over a range of stagnation enthalpies. Both the inviscid and viscous drag were seen to be relatively insensitive to stagnation enthalpy, with the combustion chambers making a particularly significant contribution to drag. The maximum fuel specific impulse achieved in the experiments was only 175 s, but the theory indicates that there is considerable scope for improvement on this through aerodynamic design.

Test flow disturbances in an expansion tube

The operation of an expansion tube is investigated with particular attention given to the test flow disturbances which have limited their utility in the past. Theoretical bounds for the duration of uniform test flow are first explored using one-dimensional ideal-gas relations, together with shock-tube boundary-layer entrainment effects. It is seen that test flow duration is limited either by the arrival of the downstream edge of the test-gas unsteady expansion or by the arrival of the upstream edge of this expansion after it has been reflected from the driver-test gas interface. These bounds are seen to be in good agreement with measurements made with large driver-gas expansion ratios. For small expansion ratios additional disturbances are observed in the test gas. Similar disturbances are also observed in the driver gas. It is postulated that these disturbances first appear in the driver gas and are transmitted into the test gas before the test gas is expanded. These disturbances remain with the test gas as it is expanded and subsequently produce unsteady conditions at the test section. Theoretical calculations for the range of frequencies which occur in the test gas before the expansion are obtained by modelling the disturbances as acoustic waves. It is shown that only the high-frequency components of the disturbances in the driver gas can penetrate the driver-test gas interface and this provides a mechanism for suppressing disturbances in the test gas. Additional analytical calculations for the shift in frequency produced as an acoustic wave traverses an unsteady expansion are also presented and it is shown that all frequencies of a given acoustic wave mode converge to one frequency. This focusing of frequencies is seen to occur in three different facilities.

Measurements of heat transfer to a flat plate in a dissociated high-enthalpy laminar air flow

Heat-transfer rates from a non-equilibrium hypersonic air flow to flat plates at zero and 12° incidence have been measured in a free piston shock tunnel at stagnation enthalpy levels up to 51 MJ kg. Nozzle flow conditions resulted in test section velocities up to 8·1 km 8 and in an experimental regime in which the free stream was chemically frozen and the flat-plate boundary layer was laminar. Estimates of the gas-phase and surface-reaction Damkohler numbers have been made and the heat-transfer results are discussed in this context. At the highest test-section densities non-equilibrium endothermic gas phase reactions involving oxygen atoms in the boundary layer are suggested as a possible mechanism for the observed low heattransfer rates.

Control of Hypersonic Turbulent Skin Friction by Boundary-Layer Combustion of Hydrogen

Shvab-Zeldovich coupling of flow variables has been used to extend Van Driests theory of turbulent boundary-layer skin friction to include injection and combustion of hydrogen in the boundary layer. The resulting theory is used to make predictions of skin friction and heat transfer that are found to be consistent with experimental and numerical results. Using the theory to extrapolate to larger downstream distances at the same experimental conditions, it is found that the reduction in skin-friction drag with hydrogen mixing and combustion is three times that with mixing alone. In application to flow on a flat plate at mainstream velocities of 2, 4, and 6 knits, and Reynolds numbers from 3 X 10(6) to 1 x 10(8), injection and combustion of hydrogen yielded values of skin-friction drag that were less than one-half of the no-injection skin-friction drag, together with a net reduction in heat transfer when the combustion heat release in air was less than the stagnation enthalpy. The mass efficiency of hydrogen injection, as measured by effective specific impulse values, was approximately 2000 s.

Hypervelocity Skin-Friction Reduction by Boundary-Layer Combustion of Hydrogen

Shock-tunnel measurements of Stanton number and skin-friction coefficient are reported for the injection of hydrogen through a 1.6-mm slot into a turbulent boundary layer in a l-m-long duct. The mainstream Mach number of 4.5, stagnation enthalpy of 7.8 MJ/kg, pressure of 50 kPa, and temperature of 1500 K provided a combination of flow variables that was sufficient to ensure boundary-layer combustion of the hydrogen. The experiments were also simulated by a numerical mode). The experiments and the numerical model indicated that the Stanton number was only slightly affected by boundary-layer combustion. However, the numerical simulation indicated that injection with combustion caused a reduction of approximately 50% in the skin friction coefficient, whereas the experiments yielded an even greater effect, with the reduction in skin-friction coefficient reaching 70-80% of the values of skin friction with no injection. Numerical simulation of a constant pressure flow indicated that boundary-layer combustion caused the skin-friction reduction to persist for at least 5 m downstream.

Comparison of Supersonic Combustion Between Impulse and Vitiation-Heated Facilities

A comparison has been made between supersonic combustion in two commonly used, but fundamentally different, facilities for scramjet research, a vitiation-heated blowdown tunnel and a free-piston shock tunnel. By passing the shock-tunnel freestream flow through a normal shock and then expanding it to Mach 2.5, combustor inlet conditions and geometries were nominally replicated between the two facilities. A constant-area rectangular duct and a diverging duct, both employing central-strut hydrogen injection, were used. Boundary-layer separation and choking in the constant-area duct limited supersonic combustion comparisons up to a fuel equivalence ratio of the order of 0.3. The experimental results also show that the onset of boundary-layer separation occurs at the same combustor pressure loads and that it behaves similarly in the different facilities. With the diverging duct, comparisons were made up to an equivalence ratio of 1.05. Agreement between the results obtained in the two facilities is within experimental error when the different freestream and boundary layers are accounted for.

Supersonic hydrogen combustion with a short thrust nozzle

Abstract A shock tunnel was used to investigate the thrust produced by supersonic combustion of hydrogen in a simple, two-dimensional combustion chamber-exhaust nozzle combination, at stagnation enthalpies up to 15 MJ kg−1. The experiments demonstrated that one-dimensional flow concepts were not adequate to predict the thrust level, and that two-dimensional effects must be taken into account. An analysis was developed which allowed the internal thrust increment of the two-dimensional nozzle to be compared with that of an equivalent one-dimensional one. It was found that, up to combustor static temperatures of 2000K, measured thrust increments were approximately half of the value for premixed equilibrium flow through the equivalent one-dimensional nozzle. Ignition occurred at approximately 1000K, and values of internal specific impulse rose above 1000 s just above this temperature, but fell to 500 s as temperatures rose above 2000K.

Release of chemical energy by combustion in a supersonic mixing layer of hydrogen and air

The process involved in chemical energy release by combustion in a supersonic, constant pressure, hydrogen-air laminar mixing layer was studied computationally, with a chemical kinetics model involving nineteen reactions and eight species. To try to find out the physical reason for the different trends of the pressure curves observed in an experimental supersonic combustor at two different initial air stream temperatures. Two initial air stream temperatures corresponding to the two experimental cases are chosen such that the higher temperature yielded a shorter ignition distance, and the lower temperature yielded a longer ignition distance. For both cases the stream wise rate of energy release rises rapidly to a peak after ignition then falls to a post-ignition value which decreases very slowly with distance. A single premixed flame occurs at ignition for both cases, but then develops into a triple flame structure in the high temperature case, and a flame with only two branches in the low temperature case. The flames move from the airside to hydrogen side consuming the oxygen as they go, until the post-ignition phase is reached. There the dominant energy release arises from the formation of a diffusion flame. In the high temperature case a narrow lean premixed flame accompanies this diffusion flame on the airside. The flame structure, but not the energy release, is effected by the initial temperature distribution across the mixing layer, which is found to be influenced by the velocity difference between the faster air stream and the slower hydrogen stream. Increasing the concentration of oxygen atoms in the oncoming air stream was found to cause substantial reduction in the ignition distance, but did not significantly effect the flame structure, or the rate of heat release in the post-ignition phase. Finally, the different trends of pressure curves observed in the experiment can be reconstructed when pressure variation was considered in this model. Thus we can conclude that the difference in the trends of the pressure curves is caused by the difference in the initial air stream temperature.

GLANCING INTERACTIONS BETWEEN SINGLE AND INTERSECTING OBLIQUE SHOCK WAVES AND A TURBULENT BOUNDARY LAYER.

The three-dimensional interactions of weak swept oblique shock and expansion waves and a turbulent boundary layer on a flat plate are investigated. Upstream influences in a single swept interaction are found to be consistent with a model of the flow involving shock/boundary-layer interaction characteristics. The model implies that there is more rapid thickening of the boundary layer close to the shock generator and this is seen to be consistent with surface streamline patterns. It is also found that a superposition principle, which is inherent in the triple-deck model of shock/boundary-layer interactions proposed by Lighthill, can be used to predict the pressure field and surface streamlines for the case of intersecting shock interactions and for the intersection of a shock with a weak expansion.

Shock-tunnel skin-friction measurement in a supersonic combustor

Shock-tunnel measurements are reported of skin friction with supersonic hydrogen-air combustion in a constant area duct. A floating-element skin-friction gauge was used, in which the shear force was applied directly to a piezoceramic measuring element. The experiments were conducted at stagnation enthalpies of 5.7 and 6.8 MJ kg(-1), a precombustion Mach number of similar to 4.5, and with a maximum duct Reynolds number of 1.3 x 10(7). The measurements showed that, although supersonic combustion caused the skin friction to fluctuate with time, it did not affect the mean value of the skin friction coefficient, and this mean value could be predicted using existing turbulent: boundary-layer theory. Measurements of heat transfer also established that Reynolds analogy could be used in both the fuel-off and fuel-on flows.