Flow Characterization of Porous Ultra-High-Temperature Ceramics for Transpiration Cooling

Abstract

Porous Ultra-High-Temperature-Ceramics (UHTC) are a candidate group of materials for transpiration cooling of hypersonic vehicles due to their exceptionally high melting point, typically above 3000 K. Their high operating temperature permits a higher amount of radiative cooling than that achievable with conventional materials, which reduces the required coolant mass flow rate to cool the surface. This work experimentally examines the internal and external flow behaviour of porous UHTC made of zirconium diboride (ZrB 2 ) for the purpose of transpiration cooling. A dedicated ISO standard permeability test rig was built. The outflow velocity distribution was acquired employing miniature hot-wire anemometry. The data obtained for the pressure loss across the porous samples agree with the Darcy-Forchheimer model for flow in porous media; respective Darcy and Forchheimer permeability coefficients are calculated and reported. Cleaning the surface of the samples using sandpaper or an ultrasonic bath raised the permeability coefficient by up to 19%. The outflow velocity maps exhibit a good flow uniformity with an average standard deviation of 25.1% with respect to the mean value.Individual jets are absent, and the velocity varies within the same order of magnitude.

Laminar transpiration cooling experiments in hypersonic flow

“…A two-dimensional (2D) flow can only be assumed within this 'V' shaped zone (see "Appendix 4") due to the aspect ratio of the model. In contrast to the non-uniform film coverage discussed in Ifti et al (2019), the film trace in the current results does not demonstrate noticeable non-uniformities. .6 for 𝜉 > 0.96.…”

“…This is expected because the injector (UHTC-7, see Fig. 3) employed in the current experiments has a significantly more uniform (Ifti et al 2022, standard deviation of 25.1% , see) outflow than the one used in Ifti et al (2019).…”

“…Note: all dimensions are in mil-limetres. b A scanning electron microscopy (SEM) micrograph of the microstructure of the UHTC injector (Ifti et al 2022) x = 168 mm , x = 250 mm , and x = 332 mm . In addition to measuring heat flux, the thin-film arrays were employed to determine the boundary-layer state, i.e.…”

“…The outflow distribution of the employed porous injector was characterised by hot-wire anemometry with air injection. Details of the process can be found in Ifti et al (2022). The obtained velocity map is illustrated in Fig.…”

“…Substituting Eqs. ( 12), ( 14), ( 15), ( 16), and p out = p e in the integrated version of the Darcy-Forchheimer equation (Ifti et al 2022) yields a form of the Darcy-Forchheimer equation that is directly dependent on the blowing ratio and unit Reynolds number given as follows:…”