Vedat S. Arpaci

Stability of natural convection in a vertical slot

The stability of natural convection of a viscous fluid in a vertical slot having isothermal side walls of different temperatures is investigated analytically. Both the conduction and boundary-layer regimes are found to be unstable with respect to stationary disturbances in the form of multicellular secondary flows. Theoretical predictions of the critical Rayleigh number and of the form of the secondary flow are verified by experimental measurements.

AN EXPERIMENTAL INVESTIGATION OF TURBULENT NATURAL CONVECTION IN AIR AT LOW PRESSURE ALONG A VERTICAL HEATED FLAT PLATE

Abstract An experimental investigation of turbulent natural convection in air is described. The results of this study show good agreement with early investigations and remarkable agreement with the analytical correlation of Bayley [4]: Nu = 0·10 Ra 1 3 for Rayleigh numbers up to 1012. Extensive measurements of the temperature field indicate a good similarity in the temperature profiles when compared on the basis of the natural coordinate ty. The use of power law temperature profiles is shown to be undesirable for the case of turbulent natural convection.

Effect of thermal radiation on the laminar free convection from a heated vertical plate

An analytical attempt is made to understand the non-equilibrium interaction between thermal radiation and laminar free convection in terms of a heated vertical plate in a stagnant radiating gas. The effect of radiation is taken into account in the integral formulation of the problem as a one-dimensional heat flux, evaluated by including the absorption in thin gas approximation and the wall effect in thick gas approximation. The local Nusselt numbers thus obtained help to interpret the gas domains from transparent to opaque and from cold to hot. The present thick gas model approximates the radiant flux as qR = −16σ3α1−1 −ϵw2exp (− 32αy)T3∂T∂y whose limit for large α and small but non-zero y is the Rosseland gas, qRRs = − (16σ3α) T3(∂T∂y), and that for y = 0 and large but finite α is qRw = − αw(8σ3α) T3w(∂T∂y)w.

Prediction of spark kernel development in constant volume combustion

Abstract Combustion initiation is studied in atmospheric pressure propane-air mixtures in a constant volume bomb with a high speed (10,000 fps) laser schlieren system. The spark current and voltage waveforms are simultaneously recorded for later model input. A phenomenological model for early flame kernel development is presented which accounts for the initial, breakdown generated, spark kernel and its subsequent growth. The kernel growth is initially controlled by the breakdown process and the subsequent electrical power input. A new, spark power induced, mass entrainment term is shown to model this initially rapid volume increase adequately while later growth is mainly dominated by diffusion. Results and model comparions are presented for the effects of power input, spark energy, and equivalence ratio.

Spark ignition of propane-air mixtures near the minimum ignition energy: Part I. An experimental study

Kernel growth from a spark in propane-air mixtures at atmospheric pressure is studied in a constant volume bomb with a high-speed laser schlieren system. The spark current and voltage waveforms of an inductive ignition source are simultaneously recorded with the photographic recordings. The temporal growth of the measured equivalent radii at conditions near the minimum ignition energy shows the existence of a critical radius and the influence of the critical radius on kernel development. In addition, it is shown that the net spark power for ignition can be estimated using data from minimum ignition energy, electrode fall energy losses, and spark calorimetry experiments. These results are used in Part II to develop a model for kernel growth.

Thermal stability of radiating fluids: The Bénard problem

The Benard problem of the radiating nongray fluids is examined in terms of the Eddington approximation. The nongrayness of radiation is prescribed by the ratio and product of the Planck and Rosseland means of the absorption coefficient, η = (αP/αR)1/2 and αM = (αPαR)1/2, respectively. Effects of radiation on the classical problem are then characterized by four parameters: the Planck number, P0 (the ratio of conduction to radiation), optical thickness, τ = αMd (d being the distance between the plates) nongrayness of the fluid η and the emissivity of boundaries e0 and e1, respectively. The radiation in general has a stabilizing effect; decreasing P0, increasing degree of nongrayness for η > 1, changing color of boundaries from black to mirror all delay the onset of instability. The boundary color and nongrayness of gas are responsible for the extrema observed in stability curves. Accuracy of the Eddington approximation is checked with the exact solution and the convergence of the approximate solution is stu...

Heat transfer enhancement in the oscillating turbulent flow of a pulse combustor tail pipe

Abstract Heat transfer rates in pulse combustor tail pipes and in other reversing, oscillating, turbulent flows have been found to be much higher than those of steady turbulent flow. To elucidate the mechanisms of the enhancement, the temperature and velocity fields, measured with two-line atomic fluorescence (TLAF) and laser Doppier velocimetry (LDV), respectively, are compared. Time-resolved wall heat fluxes and Nusselt numbers are also presented and discussed. Possible causes for the heat transfer enhancement in oscillating flows are reviewed and discussed in view of the data presented in this paper and the recent literature.

Entropy production in flames

Abstract Thermodynamic foundations of the thermal entropy production are rested on the concept of lost heat, ( Q T ) δT. The thermomechanical entropy production is shown to be in terms of the lost heat and the lost work as δS 8 = 1 T Q T δT+δW l where the second term in brackets denotes the lost (dissipated) work into heat. The dimensionless number Πs describing the local entropy production s‴ in a quenched flame is found to be Πs∼(Ped0)−2, where Π s = s‴l 2 k , l = α S u 0 (a characteristic length), k thermal conductivity, α thermal diffusivity, Su0 the adiabatic laminar flame speed at the unburned gas temperature, Pe d 0 = S u 0 D α the flame Peclet number, and D the quench distance. The tangency condition ∂ Pe d 0 ∂θ p = 0 , where θ b = T b T b 0 , Tb and Tb0 denoting, respectively, the burned gas (nonadiabatic) and adiabatic flame temperatures, is related to an extremum in entropy production. The distribution of entropy production between the flame and burner is shown in terms of the burned gas temperature and the distance from the burner.

The thermal behavior of thin metal films in the hyperbolic two-step model

Abstract A new approach, based on the physical decoupling of the hyperbolic two-step model, is introduced to describe the thermal behavior of a thin metal film exposed to picoseconds thermal pulses. The approach is based on the assumption that the metal film thermal behavior occurs in two separate stages. In the first stage, electron gas transmits its energy to the solid lattice through electron–phonon coupling and other mechanisms of energy transport are negligible. In the second stage, electron gas and solid lattice are in thermal equilibrium, the energy transfer through electron–phonon coupling is negligible, and thermal diffusion dominates. The proposed approach eliminates the coupling between the energy equations and the reduced differential equations are easier to handle. The proposed approach applies to metal films whenever the dimensionless parameter GL2/Ke is much larger than one.

Entropy production in boundary layers

The development of the entropy production in moving media entails consideration of momentum, energy, and entropy balances. The radiation-affected forced convection over a flat plate is presently investigated in terms of a thin gas, with a view to evaluating the distribution of entropy within and outside the radiation-affected thermal boundary layer. The retained nonlinearity of temperature in the entropy production leads to an extremum in this production within the boundary layer, rather than on the boundary. 14 refs.