Heat transfer reduction

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... 

Siloxanes, nevertheless, are used in small amts dissolved in N204 to reduce heat transfer thru rocket motor walls during combustion. The addn of 0.6 to 1.8% siloxane produces a heat-transfer reduction of greater than 30% (Ref 35a). Also, in the patent of Gordon et al (Ref 35b), rocket motor combustion stability is provided by the addn of a siloxane. A propint with a siloxane additive was found to bum with a nearly constant press of 600psig for approx 6 secs, whereas the same propint stabilized with cellulose acetate exhibits two press extremes and a continuously decreasing press after less... 

CA 80, 97051 (1974) 3Sa) K. Berman et al, Additives for Heat-Transfer Reduction in the Propellant Combinations Nitrogen Tetroxide-MMH [Monomethylhydra-zine] and Nitrogen Tetroxide-A-50 , JSpacecr-Rockets 1973, 10 (8), 493-95 CA 82, 127132... 

EXAMPLE 8-9 Heat-transfer reduction with shield... 

Numerical procedure to calculate the heat transfer during evaporation has heen developed for the rectangular minichannel. Some conductive calculations are shown to compare with the experimental data. They illustrate the non-uniform nature of local heat transfer around the perimeter of the channel. Rivulet and dry spot formation are descrihed. Numerical calculations show the enhancement of the heat transfer at large liquid Reynolds numbers caused at least in part by liquid suction to channel s corner where the area of microscale heat transfer arises. At small liquid Reynolds numbers the heat transfer reduction occurs due to dry spot formation. By comparing the model with the experimental data, it is shown that these conclusions are in consistent with the experimental results. 

In addition to reduced friction factors (reduced momentum transfer), the heat transfer abilities of DR solutions are also greatly reduced. This may be caused by the thickened viscous boundary layers of DR flow and/or by reduced velocity fluctuations perpendicular to the flow.f Heat transfer reduction is defined as ... 

Heat transfer reduction can be beneficial or detrimental to the practical use of DR solutions in DHC systems. On the beneficial side, the cost of insulation... 

Fig. 15 Heat transfer reduction/DR ratio for a number of surfactant solutions. (From Ref. )...

Aguilar, G. An experimental study of drag and heat transfer reductions in turbulent pipe flows... 

While the stress tensor component tfor purely viscous fluids can be determined from the instantaneous values of the rate of deformation tensor 4, the past history of deformation together with the current value of 4, may become an important factor in determining t, for viscoelastic fluids. Constitutive equations to describe stress relaxation and normal stress phenomena are also needed. Unusual effects exhibited by viscoelastic fluids include rod climbing (Weis-senberg effect), die swell, recoil, tubeless siphon, drag, and heat transfer reduction in turbulent flow. 

FIGURE 10.35 Comparison of percentage friction reduction and percentage heat transfer reduction. 

The solid line in the figure represents the general trend of the experimental observations of Refs. 35 and 85, confirming the fact that the heat transfer reduction always exceeds the friction factor reduction. This contradicts the common assumption of the validity of the Reynolds or Colburn analogy made in a number of heat transfer studies of viscoelastic fluids [101-106],... 

In terms of their effect on heat exchanger performance the measurement of heat transfer reduction or increase in pressure drop provide a direct indication. The simple methods of measuring deposit thickness described earlier are useful, but in general they require that the experiment is terminated so as to provide access to the test sections. Ideally non-intrusive techniques would allow deposition to continue while the experimental conditions are maintained without disturbance. Such techniques include the use of radioactive tracers as already referred to earlier... 

Heat transfer reduction, also expressed as a percentage, is generally obtained by comparing the Nusselt number (a dimensionless number defined as the ratio of convective to conductive heat transfer across a boundary) of a DR surfactant solution (Nu) with that of the solvent (Nu. ) at the same solvent Reynolds number ... 

A four-tube pass shell and tube heat exchanger heats 2.5 kg/sec of water from 25 to 70°C in the tubes. The heating fluid is water at 93°C (flow rate of 5 kg/sec). Overall heat transfer coefficient is 800 W/m °C. If both the overall heat transfer coefficient and the hot fluid rate are constant, find the percent heat transfer reduction as a function of cold fluid mass flow rate. 

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