Transfer, enhancement

A significant heat-transfer enhancement can be obtained when a nonckcular 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 nonckcular tube (23,24) and to the temperature-dependent non-Newtonian viscosity (25). Using an aqueous solution of polyacrjiamide the laminar heat transfer can be increased by about 300% in a rectangular duct over the value of water (23). 

For normalization of the value of the heat transfer enhancement, we used its magnitude at the maximum for each curve. The result of such normalization is shown in Fig. 2.59. In this figure, C is the solution concentration, Cq is the characteristic concentration, h is the heat transfer coefficient at given values of the solution concentration and the heat flux q, /zmax is the maximum value of the heat transfer coefficient at the same heat flux, and /zw is the heat transfer coefficient for pure water at the same heat flux q. Data from all the sources discussed reach the same value of 1.0 at the magnitude of relative surfactant concentration equal to 1.0. 

Confined boiling of water and surfactant solutions under condition of natural convection causes a heat transfer enhancement. Additive of surfactant leads to enhancement of heat transfer compared to water boiling in the same gap size however, this effect decreased with decreasing gap size. For the same gap size, CHF decreases with an increase in the channel length. CHF in surfactant solutions is significantly lower than in water. 

Yang YM, Maa JR (2003) Boiling heat transfer enhancement by surfactant additives. In Proceedings of the 5th International Conference Boiling Heat Transfer, ICBHT, Montego Bay, Jamaica, 4-8 May 2003... 

The average Nusselt number, Nu, is presented in Fig. 4.10a,b versus the shear Reynolds number, RCsh- This dependence is qualitatively similar to water behavior for all surfactant solutions used. At a given value of Reynolds number, RCsh, the Nusselt number, Nu, increases with an increase in the shear viscosity. As discussed in Chap. 3, the use of shear viscosity for the determination of drag reduction is not a good choice. The heat transfer results also illustrate the need for a more appropriate physical parameter. In particular. Fig. 4.10a shows different behavior of the Nusselt number for water and surfactants. Figure 4.10b shows the dependence of the Nusselt number on the Peclet number. The Nusselt numbers of all solutions are in agreement with heat transfer enhancement presented in Fig. 4.8. The data in Fig. 4.10b show... 

Kostic M (1994) On turbulent drag and heat transfer reduction phenomena and laminar heat transfer enhancement in non-circular duct flow of certain non-Newtonian fluid. Int J Heat Mass Transfer 37 133-147... 

Haedt, S., Ehefeld, W., Hessel, V., VANDEN Bussche, K. M., Strategies for size reduction of microreactors by heat transfer enhancement effects, in Proceedings of the 4th International Conference on Microreaction Technology, IMRET 4, pp. 432-440 (5-9 March 2000), AIChE Topical Conf Proc., Atlanta, USA. 

In the articles cited above, the studies were restricted to steady-state flows, and steady-state solutions could be determined for the range of Reynolds numbers considered. Experimental work on flow and heat transfer in sinusoidally curved channels was conducted by Rush et al. [121]. Their results indicate heat-transfer enhancement and do not show evidence of a Nusselt number reduction in any range... 

Figure 2.35 Cross-section through a staggered arrangement of micro fins designed for heat transfer enhancement in a micro channel (above) and ratio of Nusselt and Poiseuille numbers as a function of air flow per unit area for different total fin lengths (below), taken from [127].

This observation is supported by experiments carried out with a phosphate buffer and a fluorescein solution for visualization of the mixing process. However, in the experiments there are indications that the critical Reynolds number where the mass transfer enhancement sets in is lower (-7) than predicted by the simulations, a fact which is not well understood. 

In chemical micro process technology there is a clear dominance of pressure-driven flows over alternative mechanisms for fluid transport However, any kind of supplementary mechanism allowing promotion of mixing is a useful addition to the toolbox of chemical engineering. Also in conventional process technology, actuation of the fluids by external sources has proven successful for process intensification. An example is mass transfer enhancement by ultrasonic fields which is utilized in sonochemical reactors [143], There exist a number of microfluidic principles to promote mixing which rely on input of various forms of energy into the fluid. 

A main objective of the work of Hardt et al. was to study the influence of heat transfer on the achievable molar flux per unit reactor volume of the product species. They compared unstructured channels to channels containing micro fins such as shown in Figure 2.31. Heat transfer enhancement due to micro fins resulted in a different axial temperature profile with a higher outlet temperature in the reaction gas channel. Owing to this effect and by virtue of the temperature dependence... 

Performing this reaction primarily served as a model to show the feasibility of micro flow processing for soHd/Hqnid reactions [19]. In a similar way as for catalyzed gas-phase reactions, micro-reactor processing was expected to show benefits in terms of mass and heat transfer. Particularly this relates to transfer enhancement when using porous media. 

Thermal conductivity enhancement in solid reactor bed becomes one of important subjects. Enhancement of reactor bed thermal transfer using mixture of reactants and heat transfer enhancer were discussed in France... 

Thus, for heat transfer enhancement to solve the problem without installation of new area ... 

If heat transfer enhancement is only for the inside of the tubes using inserts, then ... 

Equation 15.78 gives the criterion for heat transfer enhancement to cater to the new duty without increasing the heat transfer area. If it is assumed that the resistance to heat transfer across the tube wall is negligible (i.e. hw goes to infinity) and the difference between the inside and outside diameters is negligible (i.e. d0 = df), then Equation 15.78 simplifies to9 ... 

The major disadvantage in using heat transfer enhancement is that it increases the pressure drop. In retrofit, this can be important, as the pumps driving the flow might be limited in their capacity to meet the required increase in pressure drop. 

The heat transfer enhancement ratio can now be calculated from Equation 15.78 ... 

On the basis of heat transfer enhancement and pressure drop considerations, Twisted Tape H/d/8.5 would be chosen. However, a detailed examination of capital cost might cause this to be revised. 

Zhu XX, Zanfir M and Klemes I (2000) Heat Transfer Enhancement for Heat Exchanger Network Retrofit, Heat Transfer Eng, 21 7. 

Polley GT, Reyes Athie CM and Gough M (1992) Use of Heat Transfer Enhancement in Process Integration, Heat Recovery Syst CHP, 12 191. 

Also, as discussed in Chapter 15, rather than install additional heat transfer area to cater for the new operational requirements, heat transfer enhancement can be considered. Changes to the number of tube passes or the baffle arrangement might allow the heat transfer coefficient to be enhanced. Alternatively, tube inserts could be used. This was discussed in Chapter 15. The major disadvantage in using heat transfer enhancement is that it increases the pressure drop. In retrofit this can be important, as the pumps driving the flow might be limited in their capacity to meet the required increase in pressure drop. 

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