Instantaneous temperature

Mixing Cell Calorimetry (MCC) The MCC provides information regarding the instantaneous temperature rise resulting from the mixing of two compounds. Together, DSC and MCC provide a reliable overview of the thermal events that may occur in the process. 

The simplest method that keeps the temperature of a system constant during an MD simulation is to rescale the velocities at each time step by a factor of (To/T) -, where T is the current instantaneous temperature [defined in Eq. (24)] and Tq is the desired temperamre. This method is commonly used in the equilibration phase of many MD simulations and has also been suggested as a means of performing constant temperature molecular dynamics [22]. A further refinement of the velocity-rescaling approach was proposed by Berendsen et al. [24], who used velocity rescaling to couple the system to a heat bath at a temperature Tq. Since heat coupling has a characteristic relaxation time, each velocity V is scaled by a factor X, defined as... 

In this expression. Ait is the size of the integration time step, Xj is a characteristic relaxation time, and T is the instantaneous temperature. In the simulation of water, they found a relaxation time of Xj = 0.4 ps to be appropriate. However, this method does not correspond exactly to the canonical ensemble. 

Instantaneous temperature and thermal dissipation measurements in a CH4/H2/N2 jet flame (Re = 15,200) at xld = 10 and 20. The thermal dissipation is displayed on a log scale to show the wide dynamic range. 

A bluff-body stabilized flame of CH4/H2 in air (designated HMl by Dally et al. [22]) (a) time-averaged photograph of flame luminosity, (b) time-averaged streamlines from LES, (c) instantaneous visualization of OH "luminosity" from LES, and (d) instantaneous temperature field from LES. (b and d are adapted from Raman, V. and Pitch, H., Combust. Flame, 142,329,2005. With permission.)... 

The figure shows U >. S L in this region and Da is predominantly small. At the highest Reynolds numbers the region is entered only for very intense turbulence, U > SL. The region has been considered a distributed reaction zone in which reactants and products are somewhat uniformly dispersed throughout the flame front. Reactions are still fast everywhere, so that unbumed mixture near the burned gas side of the flame is completely burned before it leaves what would be considered the flame front. An instantaneous temperature measurement in this flame would yield a normal probability density function—more importantly, one that is not bimodal. 

This equation acts as a feedback control to hold the instantaneous temperature of the atoms in the simulation, 7md> close to the desired temperature, T. If the instantaneous temperature is too high, is smoothly adjusted by Eq. (9.15), leading to an adjustment in the velocities of all atoms that reduces their average kinetic energy. The parameter Q determines how rapidly the feedback between the temperature difference rMD — T is applied to... 

The studies of recombination luminescence for samples irradiated at one temperature, 71, but then stored at another, T2, allow one, in some cases, to determine the activation energy for tunneling recombination. An instantaneous temperature jump is expected to produce no change in the distribution function over the distances between the reacting particles. When the temperature jump results only in a change of the frequency factor v but not of the parameter a in the expression for W(i ), then the ratio of the luminescence intensities for any fixed moment of time is proportional to the ratio of the corresponding v values, i.e. 

Chemical reactors are the most important features of a chemical process. A reactor is a piece of equipment in which the feedstock is converted to the desired product. Various factors are considered in selecting chemical reactors for specific tasks. In addition to economic costs, the chemical engineer is required to choose the right reactor that will give the highest yields and purity, minimize pollution, and maximize profit. Generally, reactors are chosen that will meet the requirements imposed by the reaction mechanisms, rate expressions, and the required production capacity. Other pertinent parameters that must be determined to choose the correct type of reactor are reaction heat, reaction rate constant, heat transfer coefficient, and reactor size. Reaction conditions must also be determined including temperature of the heat transfer medium, temperature of the inlet reaction mixture, inlet composition, and instantaneous temperature of the reaction mixture. 

These instantaneous temperature and velocity values can be related to values of the average fluctuating mass flux

for our experimental conditions, utilizing assumptions of the ideal gas law and fast flame chemistry. Here p and u are fluctuation values of density and velocity, respectively, Knowledge of flame properties such as p u > provides key data needed for developing improved combustion models. 

Probability density functions, or histograms, of the product of instantaneous temperature x velocity were obtained through use of this combustion probe system for a variety of downsteam and radial flame test positions. A typical histogram is shown in Fig. 3, while Fig. 4 displays the same data (as well as data for a test position further downsteam) in a "scattergram" format i.e., in a plot of velocity vs. temperature. Here, each datum corresponds to a specific shot, while the histogram bins correspond to integrated results from numbers of shots. 

The basic limitations to the overall accuracy of the data presented here lie in the Raman measurement process - inherently weak, but possessing sufficient intensity as utilized here to produce, for example, only 5-7% standard deviations for instantaneous temperature determinations in a "calibrated" premixed laminar flame (9). Further development of this light scatter-... 

Two short pathways that link the a-helical and /3-hairpin macrostates without making use of microstates with an instantaneous temperature above 488K are shown in Fig.5.1. The path shown in Fig.5.1 (upper) involves the unwinding of both ends of the helix, leaving approximately one turn of helix in the middle of the molecule. This turn then serves as a nucleation point for the formation of the /3-turn, which is stabilized by hydrophobic interactions between the side chains of Y45 and F52. The native hydrogen bonds nearest to the turn then form, after which the remainder of the native hairpin structure forms. This pathway is similar to previously proposed mechanisms for the folding of the G-peptide /3-hairpin from a coil state, which emphasize the formation of hydrophobic contacts before hydrogen bond formation [17,18, 140-143] and the persistence of the /3-turn even in the unfolded state [143]. 

This non-equilibrium state which arises due to microwave energy input results in the high instantaneous temperature (Tj) of the molecules. The Tj is not directly measurable and it must be greater than the temperature of bulk system (Tb), so as to satisfy the Arrhenius equation (k = Ae ). Therefore, Tj and not Tb ultimately determine the kinetics of the reaction and this accounts for the faster rate observed in microwave reaction. 

Here the dependence of aj on the free volume Vf allows none to be captured, so that the material time depends on the thermal history and not just the instantaneous temperature. Use of a material clock similar to this can be recognized in... 

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