Diabatic Channel Research Articles

Assessment of Engineering Turbulence Models in Buoyant Diabatic Turbulent Flow

Turbulent heat transfer in buoyancy-dominated flows is a challenging problem for computational fluid dynamics (CFD). Many authors attribute model error in these conditions to the Reynolds analogy. We leverage a brand-new direct numerical simulation database to evaluate the performance of several popular turbulence models in buoyant diabatic channel flow. We find that heat transfer results are relatively accurate, with a Nusselt number error less than 20%. However, the turbulent flow solution is very inaccurate, with wall shear overpredicted by up to 100%. This indicates significant turbulence model error in such flows. We determined that the dominant sources of model error are missing physics in the algebraic Reynolds stress framework and the simple buoyancy production term used in industrial CFD. We suggest that future modeling efforts focus on these two sources of model error. We demonstrate that the Reynolds analogy is not the dominant source of model error.

Variational formulation of non-equilibrium void fraction

A general variational formulation of the dissipative two-phase flows based on the extremization of the entropy production is developed. The entropy generation rate is written outside phase equilibrium by introducing interfacial contributions due to surface tension as well as heat and mass transfer between the two phases. Prigogine's theorem of minimum entropy production is used to estimate the steady state void fraction of the two-phase flow. The corresponding flow representation is investigated out of phase equilibrium for an annular flow, experiencing friction, surface tension effects, and interphase heat and mass transfer, within a diabatic channel. It is demonstrated that the present formulation generalises previous theories by capturing the effect of mass and heat fluxes variations, and that the widely accepted expression from Zivi represents a particular case obtained under certain simplifying assumptions. Finally, a first validation of the developed formulation of two-phase void fraction is presented for different flow conditions, heat flux from the external environment, and thermophysical properties of the refrigerant with reference to data obtained from a dedicated experimental apparatus adopting a capacitance sensor and from previous literature.

Several levels of theory for description of isotope effects in ozone: Effect of resonance lifetimes and channel couplings.

In this paper, two levels of theory are developed to determine the role of scattering resonances in the process of ozone formation. At the lower theory level, we compute resonance lifetimes in the simplest possible way, by neglecting all couplings between the diabatic vibrational channels in the problem. This permits to determine the effect of "shape" resonances, trapped behind the centrifugal barrier and populated by quantum tunneling. At the next level of theory, we include couplings between the vibrational channels, which permits to determine the role of Feshbach resonances and interaction of different reaction pathways on the global PES of ozone. Pure shape resonances are found to contribute little to the overall recombination process since they occur rather infrequently in the spectrum, in the vicinity of the top of the centrifugal barrier only. Moreover, the associated isotope effects are found to disagree with experimental data. By contrast, Feshbach-type resonances are found to make dominant contribution to the process. They occur in a broader range of spectrum, and their density of states is much higher. The properties of Feshbach resonances are studied in detail. They explain the isotopic -effect, giving theoretical prediction in good agreement with experiments for both singly and doubly substituted ozone molecules. Importantly, Feshbach resonances also contribute to the isotopic -effect, moving theoretical predictions in the right direction. Some differences with experimental data remain, which indicates that there may be another additional source of the -effect.

Transient pressure drop cross-correlation during flow boiling of R134a in parallel minichannels

This work examines the flow boiling of R134a in 0.54-mm square parallel minichannels with a particular focus on analyzing the transient pressure drop of individual channels in a wide range of heat and mass flux conditions. The flow rates are in the range from 0.1 to 0.51 g/s and heat fluxes are up to 18 kW/m2. Three different types of experiments are conducted, aiming to study the interaction between flow boiling phenomena in parallel minichannels based on the transient pressure drop measurements in each channel independently. Furthermore, cross-correlation analysis is undertaken to quantify the degree of interrelationship between neighboring minichannels. Flow boiling phenomena in a single, diabatic channel influences neighboring adiabatic channels by causing out-of-phase oscillations of single-phase pressure drops. The percentage of single-phase (ΔP) data correlated with two-phase ΔP data increases from 40% to 85% as the flow rate increases from 0.1 g/s to 0.5 g/s. Experiments with nonuniformly heated parallel minichannels show the presence of strong interaction between channels having the highest heat flux difference among a set of channels at a fixed total flow rate. As the flow rate increases and pressure drop across each channel increases, flow boiling behavior in each channel tends to be more independent from other channels. Therefore, less inter-channel interaction indicates better two-phase flow distribution between parallel minichannels, and the cross-correlation coefficient can be used as an indicator of the flow maldistribution.

Photodissociation of NO2 in the (2) B22 state: The O(D12) dissociation channel

Direct current slice and crush velocity map imaging has been used to probe the photodissociation dynamics of nitrogen dioxide above the second dissociation limit. The paper is a companion to a previous publication [J. Chem. Phys. 128, 164318 (2008)] in which we reported results for the O((3)P(J)) + NO((2)Pi(Omega)) adiabatic product channel. Here we examine the O((1)D(2)) + NO((2)Pi(Omega)) diabatic product channel at similar excitation energies. Using one- and two-color imaging experiments to observe the velocity distributions of state selected NO fragments and O atoms, respectively, we are able to build a detailed picture of the dissociation dynamics. We show that by combining the information obtained from velocity map imaging studies with mass-resolved resonantly enhanced multiphoton ionization spectroscopy it is possible to interpret and fully assign the NO images. By recording two-color images of the O((1)D(2)) photofragments with different polarization combinations of the pump and probe laser fields we also measure the orbital angular momentum alignment in the atomic fragment. We find that the entire O((1)D(2)) photofragment distribution is similarly aligned with most of the population in the M(J) = +/-1 magnetic sublevels. The similarity of the fragment polarizations is interpreted as a signature of all of the O((1)D(2)) atoms being formed via the same avoided crossing. At the photolysis energy of 5.479 52 eV we find that the NO fragments are preferentially formed in v = 1 and that the vibrationally excited fragments exhibit a bimodal rotational distribution. This is in contrast to the unimodal rotational profile of the NO fragments in v = 0. We discuss these observations in terms of the calculated topology of the adiabatic potential energy surfaces and attribute the vibrational inversion and rotational bimodality of the v = 1 fragments to the symmetric stretch and bending motion generated on excitation to the (2) (2)B(2) state.

Scaling Effects for Liquid Flows in Microchannels

Many experimental works on the forced convection through microchannels seem to show that when the hydraulic diameter is less than 1 mm, the conventional theory can no longer be considered suitable to predict the pressure drop and convective heat transfer coefficients. This conclusion seemed valid for both gas and liquid flows. Sometimes the authors justified this claim by invoking “new” micro-effects. In the last few years, this conclusion seems to be controverted by additional, more accurate experimental data. For this reason, the explanation of the experimental results obtained for microchannels in terms of friction factors and convective heat transfer in the laminar regime is sought for within the bonds of the conventional theory. In particular, this study focuses on the role of viscous heating in liquids flowing through microchannels, considering them as scaling effects. The role of the cross-sectional geometry on the viscous heating is highlighted for adiabatic and diabatic channels. Design correlations that are useful in defining the limit of significance of the most important scaling effects for microchannels, such as viscous heating and conjugate heat transfer, are also presented.

Relative Contributions of Diabatic and Adiabatic Channels to Photoisomerization and Photocyclization of Diarylethylenes

Relative contributions of diabatic and adiabatic channels to photoisomerization and photocyclization processes were measured for stilbene, 1-phenyl-2-(2-naphthyl)ethylene (Ph-2N), 1,2-di-(1-naphthyl)ethylene (1N-1N), 1-(1-naphthyl)-2-(2-naphthyl)ethylene (1N-2N), and 1,2-di-(2-naphthyl)ethylene (2N-2N) by the steady-state spectrophotometric method in solutions at room temperature. The effects of solvent nature and atmospheric oxygen on the quantum yields of these processes in 1N-1N and 2N-2N were studied.

The importance of the diabatic channels in the chemi-ionization reaction O(3P)+CH(a 4Σ−)→HCO+(X 1Σ+)+e−

We have investigated the importance of the diabatic channels in the chemi-ionization reaction O(3P)+CH(a 4Σ−)→HCO+(X 1Σ+)+e−. These channels arise due to an avoided crossing along the C–O coordinate at ∼4.7 bohr, between the 2 2Π and 1 2Π adiabatic HCO states as well as between the states that correlate to these two upon bending (4&3 2A′ and 4&3 2A″). We have estimated the nonadiabatic transition probability between these states to be of the order of ∼90%. This indicates that the diabatic channels are indeed very important.

6D vibrational quantum dynamics: Generalized coordinate discrete variable representation and (a)diabatic contraction

A new discrete variable representation (DVR) in generalized vibrational coordinates is proposed together with a new mixed diabatic/adiabatic contraction technique for the treatment of multidimensional vibrational problems up to high vibrational excitations. Formally based on the equidistant Chebyshev DVR in the grid index the new formulation is particularly suitable for multidimensional minimum energy paths. The new Z-matrix DVR proposed in this paper encompasses usual valence coordinates as well as nonlinear maps of coordinates on optimal nonequidistant grids. The pointwise numerical calculation of all kinetic energy terms avoids the algebraic derivation of specialized analytical forms of the kinetic energy adding to the flexibility of the method. With efficient truncation schemes the generalized DVR allows for a compact representation of the time-dependent wave-packet dynamics in up to six dimensions. Vibrationally adiabatic approaches to the detailed modelling of multidimensional quantum-dynamics usually are hampered by the typically large number of (avoided) crossings in dense spectra. This problem is particularly severe for discrete variable representations. A solution is provided by the new technique of diabatic rotations leading to a systematic construction of locally diabatic channels. This allows the treatment of very dense spectra where conventional truncation techniques fail. Applying the new approach to the vibrational problem of tetratomic molecules demonstrates its flexibility and efficiency. The examples of formaldehyde, ammonia, and hydrogen peroxide cover the whole range from semirigid (CH2O) to large amplitude inversion (NH3) and torsional tunnelling dynamics (H2O2). In solving the full six-dimensional vibrational eigenvalue problems for CH2O and NH3 the Z-matrix DVR shows at least comparable if not superior numerical efficiency compared with specialized techniques. In the case of H2O2 the technique of diabatic rotations and adiabatic contraction for the first time allows the treatment of the tunneling dynamics significantly above the dissociation threshold up to the fifth OH stretch overtone. The calculated decrease of the tunneling rate by about one order of magnitude agrees well with experimental observations.

Non-adiabatic wavepacket dynamics for charge-exchange processes in ion-atom collisions: application to

The collisional system is studied as a test case to assess the efficiency of a full quantum time-dependent method to calculate charge-exchange cross sections. The molecular diabatic electronic channels are calculated at the CASSCF level from the adiabatic potential energy curves and the radial coupling matrix. The elements of the collision matrix are extracted, for each value of the total angular momentum K, by Fourier transforming the post-collisional amplitudes of the wavepackets which are prepared in the entry channel and propagated on the coupled effective channels for each value of K. The results are compared with previous calculations of cross sections obtained by solving the stationary scattering equations and with recent experimental results. The close-coupling wavepacket method also provides a time-dependent picture of the collision dynamics for any K value. The kinematic isotopic effect is also examined.

State-to-state rate constants for the collisional interaction of Xe(7p), Xe(6p′), and Kr(5p′) atoms with He and Ar

One-photon laser excitation of Xe(6s[3/2]2) and Kr(5s[3/2]2) atoms that were generated in a discharge-flow reactor was used to study the collisional relaxation of the Kr(5p′[3/2]1, [3/2]2, and [1/2]1), the Xe(7p[3/2]2, [3/2]1, [5/2]2, and [5/2]3), and the Xe(6p′[3/2]1, [3/2]2, and [1/2]1) states in He and Ar. Both cw and pulsed laser excitation techniques were utilized to obtain the total deactivation rate constants and product formation rate constants at 300 K. Collisions with He mainly produce Xe* and Kr* product states with small energy defects, but the rate constants can be as large as 20×10−10 cm3 atom−1 s−1, which correspond to thermally averaged cross sections of 150 Å2. Because of the rapid collisional coupling of populations in nearly isoenergetic levels, multicomponent exponential decay of the initially produced state is frequently observed. The deactivation rate constants for Ar are smaller than for He, but the product distributions tend to be more diverse than for He, and arguments based only on energy defects are not necessarily a good guide to the favored product state(s) from Ar collisions. The magnitude of the quenching cross sections for Ar is consistent with the crossing of an entrance channel with several diabatic exit channel potentials. However, the superlarge quenching cross sections for Xe*–He to just one or two product levels require special considerations. The Kr(5p′) and Xe(6p′ and 7p) rate constants with He are discussed with respect to collisional effects upon the use of cw optical pumping to convert populations in the metastable Xe(6s[3/2]2) and Kr(5s[3/2]2) levels to the Xe(6s′[1/2]0) and Kr([5s′[1/2]0) levels.

Generation of excited state potentials from photofragment spectral lines: Fano profiles in FNO

We develop a procedure to determine excited states potentials from photofragmentation data using adiabatic energy curves. The adiabatic curves, obtained from empirically derived diabatic channel potentials, are inverted to yield a polyatomic potential energy surface which is expressed as a piecewise collection of diatomic cuts. The procedure is applied to obtaining a two-dimensional potential of the FNO molecule in the S1 excited state. The potential is more successful than available ab initio surfaces in reproducing the observed asymmetric (Fano) line shapes of the FNO S1←S0 state-specific photofragment yield spectra.

Adiabatic channel potential curves for two linear dipole rotors. I. Classification of states and numerical calculations for identical rotors

Adiabatic channel potential curves for a system of two linear dipole rotors are discussed. A general classification of states is given and a numerical procedure for calculating eigenvalues as a function of interrotor distance is formulated, both in a limited and extended basis set. A system of identical (but distinguishable) rotors is treated explicitly. Unexpectedly, the adiabatic potential curves show narrow avoided crossings which suggests the possibility of constructing diabatic channel potential curves. The validity of the adiabatic assumption for the relative motion of the dipoles is discussed.

Dominant photodetachment channels in H-

Adiabatic hyperspherical potential curves are calculated for very highly excited states of H{sup {minus}} converging to H({ital n}{le}12). These potential curves reveal surprisingly weak channel interactions, manifested as extremely diabatic multiple curve crossings. We demonstrate that only resonances associated with the {ital lowest} +'' diabatic channel within each {ital n} manifold are observed in the recent experiments at Los Alamos (Harris {ital et} {ital al}.). A simple Rydberg-dipole formula gives the positions of all dominant resonances, permitting extrapolation to arbitrarily high quantum numbers.

Diabatic hindrance of heavy-ion fusion

The fusion process for heavy nuclei is studied within a generalized formulation of the diabatic approach to collective nuclear motion. It is shown that the coupled equations of motion decouple during the approach if appropriate diabatic channels are used. Thus the mean value and the variance of the barrier are completely determined from an ensemble of diabatic channel barriers, the ensemble being defined by the initial correlations. The mean shift with respect to the adiabatic barrier and the variance are calculated from the splitting of the diabatic single-particle levels at the barrier and the initial occupation probabilities given by the pairing wavefunctions of the separated nuclei. Results are presented for 23 systems of nuclei with masses around 100. Comparison with experimental data show good agreement, in particular also for the isotopic trends. Additional effects from the destruction of pairing correlations and the diabatic screening of the fusion pocket for heavier nuclei with Z 1 Z 2 ≳ 2000 are discussed.

Diabatic shifts and fluctuations of heavy-ion fusion barriers

The fusion process for heavy nuclei is studied within a generalized formulation of the diabatic approach to collective nuclear motion. It is shown that the coupled equations of motion decouple during the approach if appropriate diabatic channels are used. Thus the mean value and the variance of the barrier are completely determined from an ensemble of diabatic channel barriers, the ensemble being defined by the initial correlations. The mean shift with respect to the adiabatic barrier and the variance are calculated from the splitting of the diabatic single-particle levels at the barrier and the initial occupation probabilities given by the pairing wavefunctions of the separated nuclei. Results are presented for 23 systems of nuclei with masses around 100. Comparison with experimental data show good agreement, in particular also for the isotopic trends. Additional effects from the destruction of pairing correlations and the diabatic screening of the fusion pocket for heavier nuclei with Z 1 Z 2 ≳ 2000 are discussed.

Modified electron transfer model for excitation functions of the type M + RX → MX + R and M+ + RX− (M = alkali, R = alkyl group, X = halogen) and M′ + N2O → MO* + N2 (M′= Ba OR Sm)

Abstract A one-dimensional model is described for the excitation functions of reactions that are initiated by an electron transfer at close range. The process is governed by a barrier in the entrance channel, the abortive reflection of trajectories at higher energies and by the competition of an adiabatic and a diabatic channel for the reactive flux. The model is fitted to measured cross sections for the (K.Rb)+CH 3 I, K+C 2 H 5 Br and (Ba.Sm)÷ N 2 O reactions and the electron transfer cross section for K+CH 3 I → K + + CH 3 I - is successfully predicted from the fitting parameters of the reactive channel.

Observation of a laser-assisted ionization of the He(2(1)S,2(3)S)+He

We report the first observation of an assisted He/sup +/ yield arising from single collisions between a He(2/sup 1/S,2/sup 3/S) atom beam on a He(1/sup 1/S) target in the presence of an intense laser field. A time-of-flight analysis shows that the pulsed laser-assisted ion signal is reproducible, energy dependent, and appears in addition to the continuous ion flux coming from a known field-free diabatic channel. From the estimated value of the diabatic ionization cross section sigma/sub d/, the order of magnitude of the assisted-ionization cross section sigma/sub a/ is given.

An Analytic Solution for the Void Fraction During Simultaneous Linear Pressure Decay and Exponential Inlet Velocity Decay in a Diabatic Channel

An approximate analytical solution is presented for the spatial and temporal distribution, in a uniformly heated channel, of the void fraction for exponentially decaying inlet velocity and for a linear time-rate-of change of system pressure. The solution, in addition to increasing the extremely limited number of analytic solutions for non-steady non-homogeneous two-phase flows, allows their use in a hybrid prediction technique to explore some complex two-phase flow situations with a minimum of computation effort and expense.