Thermal performance

The thermal performance of polymers is described in terms of both short- and long-term effects and can result from changes which are physical or chemical. Short-term performance is often expressed in terms of heat distortion temperature (HDT). This measures the temperature at which a material distorts by a certain amount under a particular stress. HDT provides less information than a plot of flexural modulus versus temperature but is convenient for quick comparisons of materials. In amorphous and unfilled crystalline materials HDT is close to T. In crystalline materials HDT can be enhanced to temperatures close to the melting point by the addition of reinforcing fibres. Table 3.1 summarises Tg, and HDT values for some reinforced and unreinforced PAEK. 

In common with other plastics the effective modulus (deflection under load) of PAEK will change as a function of time, load and temperature. Creep modulus data in the form of strain measured as a function of time and temperature under a particular stress should be available from all suppliers of PAEK. The ratio of stress to strain then 

Dibenzofuran units are formed by intramolecular radical combination. The presence of air leads to much faster rates of degradation due to the formation and decomposition of hydroperoxides  

Autoacceleration takes place in the presence of air due to the decomposition of ROOH. ROOH decomposition is classically accelerated by heat, light and metal ions such as cobalt, iron, manganese and copper which can provide electrons to the process  

We might also expect that any impurity that decomposes via radical processes more readily than PEEK would serve to initiate and accelerate degradation. 

Sometimes material is used at high temperature, and sometimes we want to improve the heat resistance of some materials. The heat resistance of a material can be characterized by the thermal deformation temperature and Vlcat softening point temperature. The thermal deformation temperature can be achieved by applying a certain stress on the material to reach a certain specified 

Inorganic whiskers have a high melting point and good heat resistance. They are dispersed in a matrix resin in a fibrous shape and play the role of a skeleton. The existence of whiskers also prevents slippage of the macromolecule in the resin, which improves the glass transition temperature and the thermal deformation temperature, as well as the rigidity. 

The thermal stability of a material sometimes is represented by the temperature of 5% decomposition and usually can be determined by a thermogravimetric method. Because inorganic fillers have a high melting point, adding inorganic fillers into resin can improve the heat resistance of the polymer. A composite material is prepared by filling calcium carbonate whiskers into PP material. The thermal decomposition temperature (5% decomposition) of the composite material increased from 366.7 to 400.6°C as the whisker content was enhanced. 

4 Theoretical Analysis of Polymer Matrix Composites Filled with Inorganic Whiskers 

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Fig. 2. A simplified material thermal performance analysis for a reentry vehicle thermal protection system where = density x surface recession thickness = total aerodynamic heat/heat of ablation ...

Carbon—carbon composites for rocket nozzles or exit cones are usually made by weaving a 3D preform composed of radial, axial, and circumferential carbon or graphite fibers to near net shape, followed by densification to high densities. Because of the high relative volume cost of the process, looms have been designed for semiautomatic fabrication of parts, taking advantage of selective reinforcement placement for optimum thermal performance. 

Moisture pickup and freeze—thaw resistance of various insulations and the effect of moisture on the thermal performance of these insulations has been reported (207). In protected membrane roofing appHcations the order of preference for minimizing moisture pickup is... 

C. J. Hilado,/ Cell Plast. 3(4), 161 (1967) I. R. ShanMand, The Effect of Cell Structure on the Buate of Foam Aging International Workshop on Long Term Thermal Performance of CeUular Plastics, SPI, Canada, Oct. 1989. 

The analytical mechanisms for predicting the corresponding pollutant formation associated with fossil-fuel-fired furnaces lag the thermal performance prediction capabiUty by a fair margin. The most firmly estabUshed mechanism at this time is the prediction of thermal NO formation (24). The chemical kinetics of pollutant formation is, in fact, a subject of research. 

Fypass Flow Effects. There are several bypass flows, particularly on the sheUside of a heat exchanger, and these include a bypass flow between the tube bundle and the shell, bypass flow between the baffle plate and the shell, and bypass flow between the shell and the bundle outer shroud. Some high temperature nuclear heat exchangers have shrouds inside the shell to protect the shell from thermal transient effects. The effect of bypass flow is the degradation of the exchanger thermal performance. Therefore additional heat-transfer surface area must be provided to compensate for this performance degradation. 

Flow Maldistribution. One of the principal reasons for heat exchangers failing to achieve the expected thermal performance is that the fluid flow does not foUow the idealized anticipated paths from elementary considerations. This is referred as a flow maldistribution problem. As much as 50% of the fluid can behave differently from what is expected based on a simplistic model (18), resulting in a significant reduction in heat-transfer performance, especially at high or a significant increase in pressure drop. Flow maldistribution is the main culprit for reduced performance of many heat exchangers. 

Although thermal performance is a principal property of thermal insulation (13—15), suitabiHty for temperature and environmental conditions compressive, flexure, shear, and tensile strengths resistance to moisture absorption dimensional stabiHty shock and vibration resistance chemical, environmental, and erosion resistance space limitations fire resistance health effects availabiHty and ease of appHcation and economics are also considerations. 

A low (<0.4 W / (m-K)) thermal conductivity polymer, fabricated iato alow density foam consisting of a multitude of tiny closed ceUs, provides good thermal performance. CeUular plastic thermal insulation can be used in the 4—350 K temperature range. CeUular plastic materials have been developed in... 

This technique reduces testing times significantly and provides reUable results for >20 years material. The values plotted in Figure 2 (25) are an illustration of the viabiUty of this technique as a means to provide reaUstic long-term thermal performance values (21). 

Polyurethane, PVC, and extruded polystyrene provide the bulk of the cellular plastics used for low and cryogenic temperature appHcations. In some cases, eg, the insulation of Hquid hydrogen tanks on space systems, foams have been reinforced with continuous glass fibers throughout the matrix. This improves strength without affecting thermal performance significantly. 

Highest thermal performance with PPS compounds requires that parts be molded under conditions leading to a high level of crystallinity. Glass-filled PPS compounds can be molded so that crystalline or amorphous parts are obtained. Mold temperature influences the crystallinity of PPS parts. Mold temperatures below approximately 93°C produce parts with low crystallinity and those above approximately 135°C produce highly crystalline parts. Mold temperatures between 93 and 135°C yield parts with an intermediate level of crystallinity. Part thickness may also influence the level of crystallinity. Thinner parts are more responsive to mold temperature. Thicker parts may have skin-core effects. When thick parts are molded in a cold mold the skin may not develop much crystallinity. The interior of the part, which remains hot for a longer period of time, may develop higher levels of crystallinity. 

Using Merkel s approximation and knowing the desired thermal performance, the flow rates, and transfer coefficient, can quickly be calculated. The difficulty with this method is that errors of >10% in can arise if the cooling range Tj — T2 is larger than a few degrees. 

Alumina is used because it is relatively inert and provides the high surface area needed to efftciendy disperse the expensive active catalytic components. However, no one alumina phase possesses the thermal, physical, and chemical properties ideal for the perfect activated coating layer. A great deal of research has been carried out in search of modifications that can make one or more of the alumina crystalline phases more suitable. Eor instance, components such as ceria, baria, lanthana, or 2irconia are added to enhance the thermal characteristics of the alumina. Eigure 6 shows the thermal performance of an alumina-activated coating material. 

The thermal performance of cylindrical rotating shell units is based upon a volumetric heat-transfer coefficient... 

Conveyor-Belt Devices The metal-belt type (Fig. 11. 55) is the only device in this classification of material-haudhug equipment that has had serious effort expended on it to adapt it to indirecl heat-transfer seiwice with divided solids. It features a lightweight construction of a large area with a thin metal wall. ludirect-coohiig applications have been made with poor thermal performance, as could be expected with a static layer. Auxihaiy plowlike mixing devices, which are considered an absolute necessity to secure any worthwhile results for this seiwice, restrict applications. 

The effective therm conductivity values generally obtained in practice are at least a factor of two greater than the one-dimensional thermal conductivity values measured in the laboratoiy with carefully controlled techniques. This degradation in insulation thermal performance is caused by the combined presence of edge exposure to isothermal boundaries, gaps, joints, or penetrations in the insulation blanket required for structure supports, fill and vent hnes, and high lateral thermal conductivity of these insulation systems. 

The ASME, Performance Test Code on Overall Plant Performance, ASME PTC 46, was designed to determine the performance of the entire heat cycle as an integrated system. This code provides explicit procedures to determination of power plant thermal performance and electrical output. 

This long-term thermal performance of a material is tested alongside a second, control, material which already has an established RTI and which exhibits a good performance. Such a control is necessary because thermal degradation characteristics are sensitive to variables in the testing programme. Since the control material will also be affected by the same unique combination of these factors during the tests, there is a valid basis for comparison of test and control materials. 

Common to all air cooled heat exchangers is the tube, through which the process fluid flows. To compensate for the poor heat transfer properties of air, which flows across the outside of the tube, and to reduce the overall dimensions of the heat exchanger, external fins are added to the outside of the tube. A wide variety of finned tube types are available for use in air cooled exchangers. These vary in geometry, materials, and methods of construction, which affect both air side thermal performance and air side pressure drop. In addition, particular... 

ISO/FDIS 13370 1998 Thermal performance of buildings—heat transfer via the ground— calculation methods. ISO, 1998. 

WINDOW A PC program for analyzing window thermal performance. Berkeley, CA Lavvrence Berkeley National Laboratory. 

Santamouris M. NORMA—a method to calculate the thermal performance of passively cooled buildings. Cooling loads of buildings, vol. 5. Dublin School of Architecture, University College Dublin, 19.94. 

Figure 3 shows the thermal performance evolution of the steam cycle as a function of material development and cycle improvements, starting in 1915. By the early 1920s, regenerative feedheating was well estab-... 

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