Hypersonic interference heating on cones with short three-dimensional protuberances

https://doi.org/10.1016/j.expthermflusci.2014.02.013Get rights and content

Highlights

  • Interference studies were conducted in shock tunnels on flat plates and cones.

  • Five enthalpy conditions ranging from 2–6 MJ/kg were tested.

  •  Effect of protrusion geometry on surface heating was studied.

  • New correlations to predict hot spot heat flux were formulated.

Abstract

Heat fluxes around short, three-dimensional protuberances on sharp and blunt cones in hypersonic flow were experimentally measured using platinum thin-film sensors deposited on macor inserts. A parametric study of different protrusion geometries and flow conditions were conducted. Excessive heating was observed at locations near the protrusion where increased vorticity is expected, with the hottest spot being presented at the foot of the protuberance immediately upstream of it. If left unchecked, these hot spots could prove detrimental to hypersonic flight vehicles. Z-type schlieren technique was used to visualize the flow features qualitatively. New correlations to predict the heat flux at the hot spot have been proposed.

Introduction

Separation of flow from the surface of aircrafts is a commonly encountered phenomenon in all flow regimes. In hypersonic flows, this may be caused by physical protuberances on the craft surface, shock impingement on the boundary layer, etc. Two-dimensional forward and backward facing steps on flat plates portray the most fundamental features such as the separation and reattachment, shear layers and recirculation zones. Early as well as contemporary experimental studies on the two-dimensional as well as three-dimensional backward facing steps have inferred the existence of regions of relatively low heat transfer within the recirculation zone and higher heat fluxes, comparable to or slightly larger than the attached flow value, at the reattachment points [1], [2]. This effort presented here attempts to quantitatively analyze the heat flux patterns in the of three-dimensional protuberances on cones, both sharp and blunt.

In realistic hypersonic aircrafts, the presence of short, three-dimensional protuberances are inevitable in the form of control surfaces, feed lines, etc. These protuberances invariably being sites of enhanced heat flux need to be properly thermally shielded. It is of equivalent importance to completely map the heat flux patterns on the surface of the craft around the protrusion caused by the interference of the flow so that adequate design changes may be adopted.

Three-dimensional protuberances are classified into two categories: short and tall [3], with different flow patterns around them, as shown in Fig. 1. Short protuberances are those which have a height comparable to the local boundary layer thickness (hδlocal). In this case, a separation shock, curved in the lateral direction, is formed upstream of the protrusion which deflects most of the boundary layer over the top of it. For most flow conditions, a recirculation region is set up at the foot of the protrusion except for cases where the deflection angle is too shallow. In tall protuberances, with hδlocal, a detached bow shock is formed immediately upstream of it leaving the protrusion engulfed in subsonic flow. The latter encounters higher pressure and heat loads as compared to the former, however, in practical designs, tall protrusions may be completely avoided.

Over the past years, experimental work has been concentrated on three-dimensional protuberances, both tall [4], [3] and short [5], [6], [7], [8], on flat plates, but recently, more attention has been addressed towards the short kind. For short protrusions, Sedney [9] proposed that a system of vortices are generated upstream of it and persist downstream by wrapping around it in a horse-shoe fashion. He observed that this generated vorticity along with the shock structures ahead of the protuberance dominated the three-dimensionality of the flow, and hence dictated the locations of enhanced heat flux in the vicinity, irrespective of the state of boundary layer or flow velocity. Studies regarding the effect of protuberances as tripping elements on the state of the local boundary layer downstream have also been conducted over the past several decades [10], [11], [12], [13]. In the realistic scenario, this is of particular interest as ablation from the surface of hypersonic aircrafts may generate protrusions or cavities, which may affect the condition of the boundary layer downstream.

Nestler [14] surveyed existing experimental data and prediction methods for interference heating due to surface discontinuities and concluded that a global correlation could not be devised due to lack of data. Estruch et al. [8], who conducted experiments on flat plate with short protrusions, concluded that the generated vortices have a large impact on the local heat flux patterns. They quantified the heat transfer rate at the hot spot by formulating predictive correlations based on their experimental data.

A similar effort was carried out by the authors of this paper to tackle the problem of interference heating on flat plates for higher enthalpy flows [15], [16]. The results showed that the existing correlations to predict the hot spot heat flux put forth by Estruch et al. [8] were not globally applicable for all flow conditions. Therefore, compiling the results from our experiments, an empirical correlation was formed for the same covering specific flow enthalpies up to 6 MJ/kg. That effort is extended in this manuscript to study the effect of interference heating on cones.

This paper presents the heat transfer data around short protuberances attached to a cone, for both sharp and blunt tip cases, at hypersonic flows with specific stagnation enthalpy ranging from 2 to 6 MJ/kg. Experiments were also conducted with varying protrusion geometries and the data was pooled together to generate an empirical correlation to predict the heat flux at the hottest spot around the protrusion. The state of current research in interference interactions over basic test models such as cones and flat plates do not extend to the higher enthalpy regime. This effort serves as the first step towards attaining that goal.

Section snippets

Shock tunnels

Two shock tunnels in the Indian Institute of Science were used for the experiments presented here; the conventional shock tunnel HST2 and the free-piston driven hypersonic shock tunnel HST3. A schematic of HST2 is shown in Fig. 2. It comprises a 2 m long driver tube and a driven tube with a length of 5.12 m. Both tubes have an internal diameter of 50 mm. A conical convergent–divergent nozzle, test section and dump tank assembly is attached to the end of driven tube. The free stream Mach number can

Background

Estruch et al. [8], in their work on interference heating on flat plates, categorized the interactions into two, (a) subcritical and (b) supercritical. For a given height of protuberance, there exists a critical lip deflection angle below which the interactions are subcritical, where no substantial recirculation zone is generated upstream of the protrusion. Supercritical interactions, in which the deflection angle is greater than the critical value, are characterized by a distinct recirculation

Conclusions

The interference heating around a three-dimensional protuberance on sharp and blunt cones were studied extensively. Experiments were conducted at five different flow conditions with flow stagnation enthalpy ranging from 2 to 6 MJ/kg, using two shock tunnels, one of the conventional type and the other, free-piston driven. The protrusion had a trapezoidal planform so as to match the fidelity of the flow direction on the cone, and its front edge was located at a distance of 143 mm from the cone tip,

Acknowledgments

This research effort was financially supported by the Indian Institute of Science (IISc) and the Defense Research and Development Organization (DRDO) of India.

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