Heat transfer history

Evaporation. Evaporative concentration can produce concentrations of 100,000 times or more in certain circumstances. Heat transfer surfaces, liquid and vapor interfaces, and regions where wetting and drying conditions occur are areas subject to evaporative concentration (see Case Histories 9.1, 9.4, and 9.6). 

FIRE SIMULATOR predicts the effects of fire growth in a 1-room, 2-vent compartment with sprinkler and detector. It predicts temperature and smoke properties (Oj/CO/COj concentrations and optical densities), heat transfer through room walls and ceilings, sprinkler/heat and smoke detector activation time, heating history of sprinkler/heat detector links, smoke detector response, sprinkler activation, ceiling jet temperature and velocity history (at specified radius from the flre i, sprinkler suppression rate of fire, time to flashover, post-flashover burning rates and duration, doors and windows which open and close, forced ventilation, post-flashover ventilation-limited combustion, lower flammability limit, smoke emissivity, and generation rates of CO/CO, pro iri i post-flashover. 

In a supersonic gas flow, the convective heat transfer coefficient is not only a function of the Reynolds and Prandtl numbers, but also depends on the droplet surface temperature and the Mach number (compressibility of gas). 154 156 However, the effects of the surface temperature and the Mach number may be substantially eliminated if all properties are evaluated at a film temperature defined in Ref. 623. Thus, the convective heat transfer coefficient may still be estimated using the experimental correlation proposed by Ranz and Marshall 505 with appropriate modifications to account for various effects such as turbulence,[587] droplet oscillation and distortion,[5851 and droplet vaporization and mass transfer. 555 It has been demonstrated 1561 that using the modified Newton s law of cooling and evaluating the heat transfer coefficient at the film temperature allow numerical calculations of droplet cooling and solidification histories in both subsonic and supersonic gas flows in the spray. 

With the above-described heat transfer model and rapid solidification kinetic model, along with the related process parameters and thermophysical properties of atomization gases (Tables 2.6 and 2.7) and metals/alloys (Tables 2.8,2.9,2.10 and 2.11), the 2-D distributions of transient droplet temperatures, cooling rates, achievable undercoolings, and solid fractions in the spray can be calculated, once the initial droplet sizes, temperatures, and velocities are established by the modeling of the atomization stage, as discussed in the previous subsection. For the implementation of the heat transfer model and the rapid solidification kinetic model, finite difference methods or finite element methods may be used. To characterize the entire size distribution of droplets, some specific droplet sizes (forexample,.D0 16,Z>05, andZ)0 84) are to be considered in the calculations of the 2-D motion, cooling and solidification histories. 

Figure 5.27 Center-line temperature history of an 8 mm thick PMMA plate during convective heating inside an oven set at 155°C. The initial temperature was 20°C. The predictions correspond to a Biot number, Bi=1.3 or a corresponding heat transfer coefficient, 7i=33 W/m2/K.[7]...

Layton, E.T., Jr. and Lienhard, J.H., Editors, History of Heat Transfer, American Society of Mechanical Engineers, New York, 1988. 

It is especially rewarding that the solutions of practical engineering problems, such as the reduction of emissions of nitrogen and sulfur oxides and polycyclic aromatic compounds from boilers, furnaces, and combustors, are amenable to the application of chemical engineering fundamentals. Guidance for preferred temperature-concentration history of the fuel may be given by reaction pathways and chemical kinetics, and elements of combustion physics, i.e., mixing and heat transfer may be used as tools to achieve the preferred temperature-concentration history in practical combustion systems. 

The rheology determines a distribution of residence time in the barrel, and the resultant heat transfer characteristics. Even with simple flow behaviour, finite-element modelling predicts greater shear rates and heating at the walls. This explains the observations by Richmond, that homogeneous melt structures first form at the wall, and further implies that on exit from the die, the melt stream may not be homogeneous with regard to its shear and/or temperature history. 

The two major costs associated with evaporators, as with any process equipment, are capital investment and operating costs. The best estimate of the installed cost of evaporation systems is, of course, a firm bid from a vendor. The installed cost, however, can be estimated based on the heat transfer surface area, as in Peters and Timmerhaus. Costs taken from published references must be adjusted for changes subsequent to the time of publication. To do this, one may use an index such as the Marshall and Swift allindustry index. The value of this index is published each month in Chemical Engineering, a McGraw-Hill publication. Further information on the use of this and other cost indices as well as their histories are available, for example, in Peters and Timmerhaus and Ulrich.f Variation of purchased evaporator costs with material of construction and pressure can also be found in Ulrich. ... 

Hemicellulose is a thermally unstable con und and is known to start to pyrolyse at 200°C [23,27,28], During a short period of the mass-loss phase the mass of birch is higher than that of pine but the char yield of pine is slightly higher. The differences are small and may be attributed to different heat transfer but because of the independence of tenq>erature (in the range 400-700 °C) it is more likely that different structure and cotiq)osition explains the difference in mass history. The evaluation of the kinetic parameters, conducted as above, shows no difference from birch (see Fig. 5). 

Sample dimension and mass should be small. For most TA techniques, samples in the form of a powder with sample mass less than 10 mg are preferred. Heat transfer between the sample and atmosphere will be faster in such a sample than in a lump thus, thermal equilibrium is more likely to be achieved between sample and atmosphere during analysis. Samples to be analyzed should have the same thermal and mechanical history. Thermal events are affected by such history and different results for the same chemical species are likely to be generated if the samples have different histories. The main reason for this is that thermal measurement is affected by the internal energy of samples, and the internal energy can be changed by thermal and mechanical processes. 

The properties which determine heat transfer through a deposit layer of given thickness are thermal conductivity, emissivity, and absorptivity. These properties vary with deposit temperature, thermal history, and chemical composition. Parametric studies and calculations for existing boilers were carried out to show the sensitivity of overall furnace performance, local temperature, and heat flux distributions to these properties in large p.f. fired furnaces. The property values used cover the range of recent experimental studies. Calculations for actual boilers were carried out with a comprehensive 3-D Monte Carlo type heat transfer model. Some predictions are compared to full-scale boiler measurements. The calculations show that the effective conduction coefficient (k/As)eff of wall deposits strongly influences furnace exit temperatures. 

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