Most fluids used in the chemical, pharmaceutical, food, and biomedical industries can be classified as non-Newtonian, ie, the viscosity varies with shear rate at a given temperature. In contrast, Newtonian fluids such as water, air, and glycerin have constant viscosities at a given temperature. Examples of non-Newtonian fluids include molten polymer, aqueous polymer solutions, slurries, coalвЂ“water mixture, tomato ketchup, soup, mayonnaise, purees, suspension of small particles, blood, etc. Because non-Newtonian fluids are nonlinear in nature, these are seldom amenable to analysis by classical mathematical techniques.
The optimum design of process equipment which handles non-Newtonian fluids could be significantly improved once predictive capability were increased. However, the basic understanding of the fluid mechanical and heat-transfer behavior of non-Newtonian, ie, viscous and viscoelastic, fluids is limited (22). A better understanding of pressure drop and heat-transfer behavior of non-Newtonian flows applicable to typical heat-exchanger geometries should lead to the design and development of more energy-efficient processes and to better quality control of the final products. In general, the viscosity of a non-Newtonian fluid can be significantly larger than that of water. Therefore, the selection of a pump size to provide enough flow rate and subsequently to ensure adequate heat removal or supply is necessary.
A significant heat-transfer enhancement can be obtained when a noncircular tube is used together with a non-Newtonian fluid. This heat-transfer enhancement is attributed to both the secondary flow at the corner of the noncircular tube (23, 24) and to the temperature-dependent non-Newtonian viscosity (25). Using an aqueous solution of polyacrylamide the laminar heat transfer can be increased by about 300% in a rectangular duct over the value of water (23).
A knowledge of the viscous and thermal properties of non-Newtonian fluids is essential before the results of the analyses can be used for practical design purposes. Because of the nonlinear nature, the prediction of these properties from kinetic theories is as of this writing in its infancy. For the purpose of design and performance calculations, physical properties of non-Newtonian fluids must be measured.
A better understanding of transport phenomena in microchannel heat exchangers appears to be vital to the development of some advanced microelectronic devices. In future designs, heat-exchanger passages are expected to be incorporated into silicon substrates for the purpose of cooling substrate-mounted microelectronic chips. The passage dimensions could be made as small (<1 [mu ]m) as those of the chip features, in which case the passage size may be comparable to the mean free path of air molecules pumped through the passages. The spacing between two molecules of gas is on the order of 1 [mgr ]m, whereas that of liquid is on the order of 0.1 [mgr ]m (13).
Further research on convective transport under low Reynolds number, quasicontinuum conditions is needed before the optimal design of such a micro heat exchanger is possible. The cooling heat exchanger is usually thermally linked to a relatively massive substrate. The effects of this linkage need to be explored and accurate methods of predicting the heat-transfer and pressure-drop performance need to be developed.
Electrohydrodynamic-Based Heat Exchangers
Electrohydrodynamics refers to the coupling of an electric field and a velocity field in a dielectric fluid continuum. Electric-field effects on heat transfer in polar gases generally take place via a modification of the gas velocity and temperature boundary layers. Electric fields in complex flows act to change the character of flow stability. Applications of electrohydrodynamics in convective heat transfer are diverse such as in heating ventilation or air conditioning (HVAC) cooling of electronic equipment applications, space power applications, micromachines, ultrasmall high duty heat exchangers, and noninvasive flow control techniques.
Characterization and influence of electrohydrodynamic secondary flows on convective flows of polar gases is lacking for most simple as well as complex flow geometries. Such investigations should lead to an understanding of flow control, manipulation of separating, and accurate computation of local heat-transfer coefficients in confined, complex geometries. The typical Reynolds number of the bulk flow does not exceed 5000.
One of the critical limitations of the increased performance of shell-and-tube heat exchangers is the onset of flow-induced vibrations at high shellside fluid flows that result in a loud acoustic (noise) vibration of more than 150 dB, or the vibration of tubes to the extent that the tube walls are worn through. Whereas a great deal of research has been done to understand vibration excitation mechanisms and to develop design guides, much is unknown about predicting flow-induced vibration occurrence, the location and type of damage, and the rates of wear. There is a substantial dependence on experience with the hardware in service, and this severely limits the development of the next generation of heat exchangers. Elimination or substantial minimization of flow-induced vibrations would have a significant impact in power, process, petroleum (qv), and other industries that use shell-and-tube exchangers.
One of the principal reasons for heat exchangers failing to achieve the expected thermal performance is that the fluid flow does not follow 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 Ntu or a significant increase in pressure drop. Flow maldistribution is the main culprit for reduced performance of many heat exchangers.
In addition to the reduction in performance, flow maldistribution may result in increased corrosion, erosion, wear, fouling, fatigue, and material failure, particularly for liquid flows. This problem is even more pronounced for multiphase or phase change flows as compared to single-phase flows. Flow distribution problems exist for almost all types of exchangers and can have a significant impact on energy, environment, material, and cost in most industries.
For gross flow maldistribution in heat exchangers, modeling is available for heat-transfer performance prediction, but no modeling is available for pressure-drop prediction. This is because, in most of the cases, the static pressure distribution is not uniform at the exchanger inlet and outlet faces, and no modeling or computational fluid dynamic analysis is possible without the boundary conditions. Gross flow maldistribution significantly increases pressure drop. In addition, because there are an infinite number of gross flow maldistributions possible, the only approach is to analyze the problem numerically for idealized uniform pressure boundary conditions.
No systematic study is reported to quantify the effect of manifold induced-flow maldistribution on a single-phase pressure drop and heat transfer in a heat exchanger. Such flow maldistribution is common in gas-to-gas and liquid-to-gas exchangers with manifolds, and in a plate heat exchanger in which many parallel passages are connected by inlet and outlet pipe manifolds created by plate ports. For two-phase flow distribution, however, no practical methods exist for ensuring the adequate distribution of the vapor and liquid phases among many parallel-flow channels. The result in the cryogenic gas processing area is, for example, that phases are separated and introduced into separate heat exchangers for further vaporization or condensation at a significant penalty in overall thermodynamic optimization of the system. Viscosity-induced flow maldistribution has been hardly analyzed to quantify the influence on heat transfer and pressure drop.
Very meager information is available in the literature on natural convection-induced flow maldistribution and its effect on the exchanger heat transfer and pressure drop. A combination of hot- and cold-fluid maldistributions, both tubeside and shellside, can create a more serious problem than the individual maldistributions alone. Heat exchangers involving multiphase flow appear to have the highest likelihood of flow maldistribution and the resulting thermal and mechanical performance loss and flow instability. This is especially critical where multiphases exist at inlet.