Heat transfer probes

For this study mixts of CeH6 O and H O were detonated in a tube either by a shock wave or by a spark. The arrival of the pressure step was detd by a thin-film, heat-transfer probe with a rise time of 0.5 microsecs. The spectrograph viewed the passing deton wave thru a window slit and lens arrangement. Recording was accomplished by photomultiplier tubes. The deton waves observed consisted of a shock front followed by a combustion front and were classed as "strong , which is equiv to "unsteady or "decelerating detonation. Detailed structure of the detonations could not be resolved... 

Many studies on the flow distribution in random packed beds have been reported in the literature. Mercandelli et al. [8] published a short review of the flow distribution work in random packed trickle bed, which includes the list of various techniques used to determine and quantify the flow distribution. Conventional methods include, for example, collecting liquid at the bottom of the column from different zones while advanced methods include tomographic techniques. Mercandelli et al. [8] used several techniques to quantify liquid distribution in columns of diameters up to 30 cm with three different distributor designs. They used global pressure drop measurements, global residence time distribution (RTD) of the liquid, local heat transfer probes, capacitance tomography and a collector at the bottom of the column. 

Measurements of heat transfer in circulating fluidized beds require use of very small heat transfer probes, in order to reduce the interference to the flow field. The dimensions of the heat transfer surface may significantly affect the heat transfer coefficient at any radial position in the riser. All the treatment of circulating fluidized bed heat transfer described is based on a small dimension for the heat transfer surface. The heat transfer coefficient decreases asymptotically with an increase in the vertical dimension of the heat transfer surface [Bi et al., 1990]. It can be stated that the large dimensions of the heat transfer surface... 

Fig. 18. Schematics of heat transfer probe (Bai et al., 1992). (a) Probe A (b) probe B.

Figure 4 Examples of heat transfer probes, (a) Low thermal capacity instantaneous type of Mickley et al. (1961) (b) miniature heat transfer probe of Ma and Zhu (1999) (c) probe to measure radiative and total heat flux. (Luan et al., 1999.) (d) radiative heat flux vs. suspension density. (Luan et al. 1999.) [(a) Reproduced with permission of the American Institute of Chemical Engineers. Copyright 1986 AIChE. All right reserved, (b)-(d) with permission of Elsevier Science]...

Figure 17 Variation of instantaneous heat transfer rate with bubble passage synchronized with the visualization of flow patterns in the vicinity of the heat transfer probe in a liquid-solid fluidized bed of low-density gel beads. (From Kumar et al., 1993a.)...

L = circumference of the ellipsoidal bubble length of the heat transfer probe... 

Figure 16. Variation of heat transfer coefficent to a 10 cm long calorimetric heat transfer probe at two different temperatures and for two different particle sizes, (Kobro and Brereton, 1986).

Of course the spectrum of quantities, which need to be measured in a fluidized bed, is much wider. These include, for example, local solids volume concentrations, solids velocities and solids mass flows, the vertical and the horizontal distribution of solids inside the system or the lateral distribution of the fluidizing gas. In response to these needs a number of more sophisticated measurement techniques were proposed. For example, suction probes were developed to measure local solids and mass flow, heat transfer probes were proposed for detection of de-fluidized zones and solids flow inside fluidized-bed reactors. Other techniques include capacitance probes, optical probes, or y-ray densitometry - a detailed review was given recently by Werther [1]. Cody et al. 2 reported the use of an acoustic probe to measure particle velocity at the wall of fluidized beds. 

Activities associated with bioreactors include gas/hquid contacting, on-hne sensing of concentrations, mixing, heat transfer, foam control, and feed of nutrients or reagents such as those for pH control. The workhorse of the fermentation industry is the conventional batch fermenter shown in Fig. 24-3. Not shown are ladder rungs inside the vessel, antifoam probe, antifoam system, and sensors (pH, dissolved oxygen, temperature, and the like). Note that coils may lie between baffles and the tank wall or connect to the top to minimize openings... 

In the case of a temperature probe, the capacity is a heat capacity C == me, where m is the mass and c the material heat capacity, and the resistance is a thermal resistance R = l/(hA), where h is the heat transfer coefficient and A is the sensor surface area. Thus the time constant of a temperature probe is T = mc/ hA). Note that the time constant depends not only on the probe, but also on the environment in which the probe is located. According to the same principle, the time constant, for example, of the flow cell of a gas analyzer is r = Vwhere V is the volume of the cell and the sample flow rate. 

Contact temperature measurement is based on a sensor or a probe, which is in direct contact with the fluid or material. A basic factor to understand is that in using the contact measurement principle, the result of measurement is the temperature of the measurement sensor itself. In unfavorable situations, the sensor temperature is not necessarily close to the fluid or material temperature, which is the point of interest. The reason for this is that the sensor usually has a heat transfer connection with other surrounding temperatures by radiation, conduction, or convection, or a combination of these. As a consequence, heat flow to or from the sensor will influence the sensor temperature. The sensor temperature will stabilize to a level different from the measured medium temperature. The expressions radiation error and conduction error relate to the mode of heat transfer involved. Careful planning of the measurements will assist in avoiding these errors. 

Probe/Insirumentalion Developments The principles of good practice in the design, construction and location of corrosion probes have been reviewed. Specific probe designs which acknowledge hydrodynamic influences and the combined effects of mass and heat transfer have been developed. 

Figure 13. Response of early warning probe heat transfer coefficient to different events during a suspension polymerization batch (19)...

Sample Introduction and Transfer System. The sample Introduction and sample transfer system is a lengthened version of the PHI Model 15-720B Introduction system which consists of a polymer bellows-covered heating and cooling probe, a transferable sample holder, an eight-port dual-axis cross, and the mlnlreactor Interface port and transfer probe (Figure 2). There Is also a transfer vessel port with the necessary transfer probe for Introduction of air sensitive samples. They are not part of the reactor/surface analysis system. The dual cross and attached hardware are supported by the probe drive mechanism which floats on a block driven vertically and transversely by two micrometers. These micrometers plus the probe drive mechanism allow X-Y-2... 

Measurement based on heat flux effects This approach uses local probing devices such as hot-wire anemometers and microthermocouples. The hot-wire anemometer can be either a constant-temperature system or a constant-heat-flux system. Because of the difference in heat transfer between the exposed fluid (liquid or gas)... 

The temperature of molten polymer process streams is commonly measured using a thermocouple positioned through a transfer line wall and partially immersed in the polymer stream. Process stream temperature measurements that use an exposed-tip thermocouple, however, can be misleading since the temperature of the thermocouple junction is a balance between the heat transferred from the polymer stream and from the thermocouple assembly [39]. Due to the low heat transfer rate between the polymer and the exposed tip and the high thermal conductivity of the thermocouple sheath, the temperatures measured can be different by up to 35°C depending on conditions. Extrudate temperatures, however, can be accurately measured using a preheated, handheld thermocouple probe. This method minimizes thermal conduction through the probe sheath. 

Probe The probe is constrncted of stainless steel and has Swagelok fittings top and bottom for attachment to the process sample line. The constrnction of the probe inclndes a Dewar that reduces the sample heat transfer throngh the walls of the probe to the magnet components. At the sampling zone the inner wall of the probe is constrncted of alnmina ceramic which is specialty welded to the stainless. The probe is pressure tested to 1.035 x 10" Pa (103.4 bar, 1500 psi). The duplexor and preamplifier are built into the base of the probe. 

In the adsorption microcalorimetry technique, the sample is kept at a constant temperature, while a probe molecule adsorbs onto its surface, and a heat-flow detector emits a signal proportional to the amount of heat transferred per unit time. 

More recently, Sole391 studied the system to determine the extent, if any, of heterogeneous reaction. By means of differential calorimetry, he compared the heat transferred to the vessel walls with that transferred to a probe in the center of the vessel. He found that heterogeneous effects could be completely disregarded. 

Research and development contracts with universities, research organizations, and industrial firms are probing into the fields of heat transfer, scale prevention, corrosion, membranes, gas hydrates, solvent extraction, and many others, all designed to help attain the goal of low-cost converted water. 

In an extension of simply monitoring and controlling dryers, Morris and colleagues have demonstrated how in situ probes can provide real-time analysis that enables optimization of a drying process.61,62 In these articles, the authors demonstrate how evaporative cooling can be used to expedite drying in formulations where heat transfer... 

Figure 12.10. Probe-to-bed heat transfer coefficient variations in a fluidized bed (from Tuot and Clift, 1973).

---