The risk involved, due to an inadequate knowledge of real gas effects, ... the heat shield surface, increase the overall
Proceedings of CHT-08 ICHMT International Symposium on Advances in Computational Heat Transfer May 11-16, 2008, Marrakech, Morocco CHT-08-330
EFFECTS OF SURFACE CATALYTICITY ON COMPUTED HEAT TRANSFER OVER A REENTRY VEHICLE A. Viviani§, G. Pezzella, C. Golia Seconda Università di Napoli, Dipartimento di Ingegneria Aerospaziale e Meccanica via Roma 29, 81031 Aversa, Italy § Correspondence author. Fax: +39 081 5010204, Email:
[email protected]
ABSTRACT This paper reports on the research activities performed by the authors for the assessment of the aeroheating environment around a reentry vehicle devoted to manned or unmanned low earth orbit missions. Since hypersonic flow conditions are such that gas dissociation occurs, the surface convective heating rate may be significantly affected by recombination at the vehicle surfaces promoted by the wall catalyticity. In this context, good capabilities of simulating the flowfield environment around the descent vehicle are fundamental to detect the high-temperature phenomena of thermochemically reacting hypersonic flows occurring at the vehicle surface. Herein, both axisymmetric and 3D Navier-Stokes equations are numerically solved for non equilibrium airflow around a capsule-type reentry vehicle at the peak heating conditions. Computed stagnation point heating rates are compared with results from the Fay Riddell and Goulard formulae, and the effect of surface catalyticity on the heat shield thermal loading is highlighted and discussed.
NOMENCLATURE a D h kw Le Mi Pr q& RN s S t u v Y
= = = = = = = = = = = = = = =
sound speed capsule diameter enthalpy catalytic reaction rate constant Lewis number molecular mass Prandtl number heat transfer rate heat shield nose radius curve length boundary layer diffusion rate entry interface time velocity along x axis velocity chemical species mass fraction
Greek symbols α = angle of attack β = energy accommodation coefficient
γ γi ε η θ λ μ ρ σ τ &a ω
= = = = = = = = = = = =
Ωμ
= collisional integral
Δh of
Subscripts a = co = e = D = i = t2 = v = w = ∞ =
specific heats ratio TPM efficiency for recombination heat of formation TPS emissivity recombination rate of atoms diffusing to wall capsule sidewall angle / flight path angle gas thermal conductivity viscosity density Stefan Boltzmann constant / collision cross section cell height species source term
atomic species stagnation point boundary layer edge conditions dissociation ith chemical species stagnation point conditions downstream a normal shock wave vibrational wall conditions freestream conditions INTRODUCTION
In the framework of designing thermal shields of reusable vehicles, a reliable prediction of reentry heat flux by means of numerical computations is ever more important. In this context, the capabilities of simulating flowfield environment around the descent vehicle should be able to account for the interaction of the high-temperature phenomena of thermo-chemically reacting hypersonic flows with the vehicle walls surface. The present work confirms that dissociated gas may increase the heat load that the vehicle heat shield has to withstand, due to the heterogeneous catalysis phenomena that eventually take place at the vehicle surface. In fact, real gas thermodynamics, transport properties and finite rate chemistry have pronounced effects on heterogeneous chemical reactions at vehicle walls which in turn affect the vehicle thermal shield performances. The risk involved, due to an inadequate knowledge of real gas effects, is that integrity and performance of the vehicle may be severely compromised due to wrong design choices as, for example, additional weight for the thermal protection system (TPS) in the case of fully catalytic wall design hypothesis. For instance, the thermal protection material (TPM) may promote the chemical recombination, at walls, of atomic species produced by dissociation in the flow when the gas passes through the strong vehicle bow shock wave [e.g., Viviani et Al. 2007a,b]. These recombination reactions, by means of the heat of formation of the molecular species leaving the heat shield surface, increase the overall heat flux up to about two times, or more, higher than the one predicted with the assumption of non-catalytic wall [Marichalar 2006]. These phenomena depend both on reentry energy and vehicle configuration; thus, the first goal of the present research is to highlight the importance of catalytic effects by simulating the flowfield around the ELECTRE probe tested in Plasma Wind Tunnel (PWT), and around an Apollo-style vehicle that is foreseen to be used as Crew Return Vehicle (CRV) for the International Space
Station (ISS) and/or as LEO (Low Earth Orbit) mission support vehicle (for example, for Hubble space telescope servicing). As second goal, since the heat shield development program for ablative TPS in moderate aeroheating environment is not yet mature, this work aims to assess the performance of a capsule forebody thermal shield built of Shuttle-like TPS tiles. To this purpose, the aeroheating environment of CRV is herein reported, with respect to both ballistic and lifting reentry trajectories. To accomplish the paper goals, two levels of numerical simulations are adopted. The first one refers to engineering analyses performed by the ENTRY tool (see below), and the second one to detailed CFD (computational fluid dynamics) analyses. The ENTRY (ENtry TRajectorY) tool is a versatile conceptual/preliminary design tool, developed at DIAM (Dipartimento di Ingegneria Aerospaziale e Meccanica), aimed to support the preliminary design phase of a reentry mission and to provide a quick reliable insight into the feasibility of a vehicle concept at an early stage of the mission design [Viviani et Al. 2006]. Instead, for the CFD analyses, both axisymmetric and 3D Navier-Stokes computations are performed with different wall boundary conditions: non catalytic wall (NCW), partially catalytic wall (PCW), fully catalytic wall (FCW).
OVERVIEW OF MODELS FOR THERMAL SHIELD CATALYTICITY In order to make a safe land, the descent vehicle has to reduce its energy (kinetic plus potential) by heating the surrounding airflow. The efficiency of this energy conversion depends on many factors and processes. The main one is the vehicle configuration and, hence, its reentry trajectory. As well stated, the most favourable condition to transfer vehicle energy into the atmosphere is attained when high pressure drag configurations, i.e. blunt bodies flying at high AoA (Angle of Attack) are employed. In this case, a strong detached shock wave forms ahead of the reentry vehicle and, due to the large total enthalpy of the freestream, the airflow behind the bow shock results into a plasma which impinges on the vehicle wall. When the atoms, resulting from the dissociation into the flow, reach the vehicle wall, some of them recombine in molecules either in the boundary layer or at the vehicle surface, where they dump their energy of recombination. This process depends on vehicle surface temperature and on flow chemical conditions, within shock and boundary layer, and provides an additional contribution to the heat transfer, due to species diffusion, which is generally of the same order of magnitude of the Fourier contribution. For instance, neglecting heat conduction into the vehicle wall and radiation from the gas, the energy balance at vehicle surface can be described in terms of two main contributions:
⎛ ∂ Yi ⎞ ⎛ ∂T ⎞ 4 q& w = −λ ⎜ ⎟ − ρ ∑i D im h Di ⎜ ⎟ = σε Tw ⎝ ∂n ⎠ w ⎝ ∂n ⎠ w
(1)
The first term is the conductive heat-flux from fluid to the wall, due to the temperature gradient, the second one is the diffusion term due to the species gradient. The latter contribution depends strongly on the surface catalytic properties of TPS. In fact, as the chemical non equilibrium flow enters the boundary layer (reacting boundary layer) the vehicle surface may acts as third body promoting heterogeneous reactions that take place at the wall. So the TPM may be involved in the surface recombination which becomes the main factor of the aerodynamic heat transfer, especially when gas phase recombination in the boundary layer is frozen and the wall absorbs the entire energy released by surface reactions (i.e. FCW with frozen boundary layer) [Fay Riddell 1958]. Therefore, in order to protect the CRV from this intense heat, the catalytic properties of the heat shield TPM candidate must be carefully known. For instance,
close to the wall, the effect of TPM on the recombination rates of heterogeneous reactions is accounted for by the catalytic efficiency coefficients γi. The recombination coefficient γi is a combination of the probability (γ’i) that atoms of the species i recombine as they collide on the surface and of the dissociation energy fraction (β) effectively released to the surface (also known as accommodation coefficient):
γ i = γ i' β
(2)
where γ’i is the ratio of the number of atoms recombining in molecules to the overall number of atoms striking on the surface:
γ i' =
& i ,recomb m & i ,all m
(3)
When γ’i =1 all atoms reaching the wall recombine and leave it as molecules: the wall is referred to as fully catalytic (FCW); on the other side, when γ’i =0 recombination does not occur and the wall is said non-catalytic (NCW). These two extreme cases are not encountered practically. Real TPMs (e.g. like C/SiC) have a finite catalyticity and γ’i takes values between 0 and 1. Silicates and ceramics are fair approximations to non catalytic surfaces while metals and metal oxides are strongly catalytic. For example, values of γi are of the order less than 0.01 on ceramic surfaces; on metallic, metallic oxide or graphitic surfaces values might be expected greater than 0.1 and are normally assumed as fully catalytic [Clark 1995]. Values of γi for nitrogen on metals can be from 0.1 to 0.2 and the metal impurities are thought to be the primary cause of surface reactivity [e.g., Kolodziej 1987]. Regarding β, experimental observations suggest that numerical modeling need to take into account an incomplete energy release at the surface:
β=
q& ,recomb & i ,recomb ⋅ h Di m
(4)
stating that the energy release due to the combined atoms differs from the total available dissociation energy of the atoms. However, for real space vehicle TPMs, very little is known about this coefficient and, for simplicity sake, β=1 is generally accepted (conservative condition), so that:
γ i = γ i'
(5)
This is the case when the residence time of the recombined molecules is long enough to transfer all the chemical energy, created by surface recombination, to the heating of the vehicle wall. One argument to support this assumption is that the energy cannot be released in the boundary layer, because of the adverse temperature gradient, and hence enters the wall, although there are some evidences that glassy materials might have partial energy accommodation, i.e. β
