Maurizio Barbato

Material-dependent catalytic recombination modeling for hypersonic flows

A new model to predict catalytic recombination rates of O and N atoms over silica re-entry thermal protection system is reported. The model follows the general approach of Halpern and Rosner, but adds estimates of some key physical mechanism parameters based on realistic surface potentials. This novel feature can therefore produce rate expressions for any surface for which structure is known. Testing the model for N over W, and N and O over SiO2 produces recombination probabilities in good agreement with published measurements at high surface temperature. In the case of N and O over SiO2, the model accounts for surface NO production due to O and N cross recombination.

Model for Heterogeneous Catalysis on Metal Surfaces with Applications to Hypersonic Flows

A model for heterogeneous catalysis for copper, nickel, and platinum has been devised. The model simulates the heterogeneous chemical kinetics of dissociated aire ow impinging metal surfaces. Elementary phenomena such as atomic and molecular adsorption, Eley ‐Rideal and Langmuir ‐Hinshelwood recombinations, and thermal desorption have been accounted for. Comparisons with experimental results for nitrogen and oxygen recombination show good agreement. The e nite rate catalysis model has been used to analyze numerically the problems of heterogeneous catalysis similarity between hypersonic ground testing and reentry e ight. Therefore, the e ow around a blunt cone under these conditions has been calculated, and results for heat e uxes and for a suggested similarity parameter have been compared and discussed. Nomenclature

State-to-State Catalytic Models, Kinetics, and Transport in Hypersonic Boundary Layers

A new iterative model has been developed that couples, in the boundary layer of a reentering body, the equations for N 2, N, O 2, O, and NO mass fractions, N 2 and O 2 vibrational distributions, and gas temperature with the surface state-to-state heterogeneous recombination coefficients has been developed. Results for SiO 2 and metallic surfaces are presented and discussed. The non-Boltzmann behavior of the vibrational distribution functions near the surface is found, as well as the nonmonotonic behavior of the NO density profile along the boundary layer coordinate. The transport coefficients and the heat flux to the surface are calculated using the Chapman-Enskog theory. A strong dependence of transport coefficients and energy flux on the vibrational-chemical kinetics in the boundary layer is shown. In particular, the diffusion coefficients of the first and last vibrational levels differ by several orders of magnitude, according to the shape of vibrational distributions, and the surface material noticeably influences diffusion coefficients of N and NO.

An Air-Based Cavity-Receiver for Solar Trough Concentrators

A cylindrical cavity-receiver containing a tubular absorber that uses air as the heat transfer fluid is proposed for a novel solar trough concentrator design. A numerical heat transfer model is developed to determine the receiver’s absorption efficiency and pumping power requirement. The 2D steady-state energy conservation equation coupling radiation, convection, and conduction heat transfer is formulated and solved numerically by finite volume techniques. The Monte Carlo ray-tracing and radiosity methods are applied to establish the solar radiation distribution and radiative exchange within the receiver. Simulations were conducted for a 50 m-long and 9.5 m-wide collector section with 120°C air inlet temperature, and air mass flows in the range 0.1‐1.2 kg/s. Outlet air temperatures ranged from 260°C to 601°C, and corresponding absorption efficiencies varied between 60% and 18%. Main heat losses integrated over the receiver length were due to reflection and spillage at the receiver’s windowed aperture, amounting to 13% and 9% of the solar power input, respectively. The pressure drop along the 50 m module was in the range 0.23‐11.84 mbars, resulting in isentropic pumping power requirements of 6.4510 4 0.395% of the solar power input. DOI: 10.1115/1.4001675 Cavity-receivers are typically used in point-focusing solar concentrating systems e.g., dishes and towers to efficiently capture incoming radiation through multiple internal reflections, while providing sufficient heat transfer area for heat removal by a heat transfer medium or by chemical reactions. In contrast, tubular receivers are typically used in line-focusing solar concentrator systems e.g., parabolic troughs to efficiently absorb incident solar radiation through the application of selective coatings and vacuum insulations. However, when the heat transfer fluid HTF has low volumetric heat capacity and thermal conductivity, as is usually the case for gases, cavity-receivers are an interesting alternative to conventional tube receivers, as they offer the potential for larger heat transfer area and flow cross section without significantly affecting the reradiation losses from the absorber. Cylindrical cavity-receivers have been previously analyzed for an annular flow cross section 1 and for a cavity containing a single absorber tube or an array of absorber tubes 2‐4. Air is used as the HTF in the present case. The advantages are fourfold: 1 Performance loss and operating temperature constraints due to chemical instability of the HTF are avoided; 2 operating pressure can be close to ambient, eliminating the need for sophisticated sealing; 3 a packed-bed thermal storage can be incorporated to the system and heated directly by air, eliminating the need for a heat exchanger between HTF and thermal storage medium; and 4 costs for the heat transfer fluid are removed. Further, by employing conventional materials of construction and avoiding selective absorber coatings, vacuum insulation, or getters, significantly lower fabrication costs per unit receiver length are expected than those for existing receivers. On the other hand, the disadvantages of air-receivers are associated with the larger mass flow rates and surface area needed due to the lower volumetric heat capacity and thermal conductivity of air as compared with those of thermo-oils, molten salts, sodium, or other heat transfer fluids proposed. These drawbacks translate into higher pressure drops and concomitant energy penalties. In this paper, a numerical heat transfer model of an air-based cylindrical cavityreceiver is developed and applied to investigate the influence of air mass flow rate on outlet air temperature, receiver’s absorption efficiency, pumping power requirements, and thermal losses.

The influence of cell morphology on the effective thermal conductivity of reticulated ceramic foams

Ceramic foams are used in thermal application both as insulations and heat exchangers. Understanding the effect of foam morphology on their effective thermal conductivity is important for design engineers. In this work, the effects of parameters such as: porosity, cell inclination angle and ligament tapering are studied using Finite Elements (FE) method. For the purpose, an algorithm for the generation and meshing of tetrakaydecahedra was implemented and steady state thermal simulations performed. Results compared with experimental data from the literature showed good agreement. A strong decrease of thermal conductivity was found by increasing porosity and ligament tapering, while a less significant increase was noticed on increasing cell inclination angle.

Comparison of catalytic wall conditions for hypersonic flow

The effects of several catalytic boundary conditions implemented in a hypersonic flow solver are analyzed for a sphere/cone geometry representative of a re-entry body. The three-dimensional Navier-Stokes equations solver uses a five-chemical-species model. The simulated surface is silica, representative of coatings for thermal protection systems. The range of wall temperatures explored is 300-1500 K, and fully catalytic, local-equilibrium, noncatalytic, and finite-rate catalysis boundary conditions are applied and discussed. For finite-rate catalysis a recent model for simultaneous recombination of O and N atoms, including NO formation, is used. A comparison of all the boundary conditions implemented with results from fit-based finite-rate catalysis boundary conditions is made at surface temperatures of 1200 and 1500 K. Numerical simulations results are compared and discussed, and conclusions about which boundary conditions are best in each case are drawn.

Surface Recombination Coefficients and Boundary-Layer Hypersonic-Flow Calculations on Different Surfaces

The interaction of dissociating oxygen and air on SiO 2 in the boundary layer on different structures was discussed. In oxygen the state-selected rates of recombination of atomic species on SiO 2 surfaces into vibrationally excited molecules in gas phase were obtained by molecular dynamic approach. The one-dimensional boundary-layer was implemented into heterogeneous catalytic-recombination coeficients depending on local flow conditions. The existence of nonequilibrium vibrational distributions near ceramic and metallic surfaces was established.

High temperature thermocline TES – effect of system pre-charging on thermal stratification

The purpose of this study is to evaluate, by means of a computational fluid dynamics approach, the effect of performing an initial charging, or pre-charging, on thermal stratification of an industrial-scale thermocline TES unit, based on a packed bed of river pebbles. The 1 GWhth TES unit under investigation is exploited to fulfill the energy requirement of a reference 80 MWe concentrating solar power plant which uses air as heat transfer fluid. Three different scenarios, characterized by 4 h, 6 h and 8 h of pre-charging, were compared with the reference case of TES system operating without pre-charging. For each of these four scenarios, a total of 30 consecutive charge/discharge cycles, of 12 h each, were simulated and the effect of TES pre-charging on thermal stratification was qualitatively evaluated, by means of a stratification efficiency, based on the second-law of thermodynamics. On the basis of the simulations results obtained, the effect of pre-charging, more pronounced during the first cycles, is not only relevant in reducing the time required by the TES to achieve a stable thermal stratification into the packed bed but also to improve the performance at startup when the system is charged for the first time.

A 3 MWth parabolic trough CSP plant operating with air at up to 650 °C

Parabolic trough concentrating solar power (CSP) has long proven to be among the most viable options for large-scale solar electricity generation. However, conventional solar parabolic trough plants suffer from several technical and economical drawbacks. These include most notably a maximum operating temperature limited to below 450 °C, a difficulty in creating rigid metallic support structures with large trough apertures, and the need for complex thermal energy storage (TES) technologies. This has impelled the development of a novel trough-based CSP system comprising a 9 m aperture parabolic trough concentrator based on inflated metallized polymer films mounted on a concrete support structure, coupled to a solar receiver based on air as heat transfer fluid and to a packed bed of rocks sensible heat storage. The first power plant with this technology, with a nominal thermal power output of 3 MWth, has been constructed in Ait Baha, Morocco. We report on the details of the system and its components.

CPV cells cooling system based on submerged jet impingement: CFD modeling and experimental validation

Concentrating photovoltaic (CPV) cells offer higher efficiencies with regard to the PV ones and allow to strongly reduce the overall solar cell area. However, to operate correctly and exploit their advantages, their temperature has to be kept low and as uniform as possible and the cooling circuit pressure drops need to be limited. In this work an impingement water jet cooling system specifically designed for an industrial HCPV receiver is studied. Through the literature and by means of accurate computational fluid dynamics (CFD) simulations, the nozzle to plate distance, the number of jets and the nozzle pitch, i.e. the distance between adjacent jets, were optimized. Afterwards, extensive experimental tests were performed to validate pressure drops and cooling power simulation results.