Inside Circular Tubes Research Articles

Effect of oil on two-phase pressure drop of refrigerant flow boiling inside circular tubes filled with metal foam

Two-phase pressure drop characteristics of refrigerant-oil mixture in circular tubes filled with metal foam were experimentally investigated. Experimental conditions cover nominal oil concentrations from 0% to 5%, metal foam structures of 5 PPI and 10 PPI, vapor qualities from 0.2 to 0.8, and mass fluxes from 30 to 90 kg/m2s. The test results show that the pressure drop increases with the increase of oil concentration, mass flux or vapor quality. The presence of oil increases pressure drop by a maximum of 25% and 50% for 5 PPI and 10 PPI metal foams, respectively. The enhancement effect of oil for high PPI metal foam is more evident than that for low PPI metal foam. A correlation for predicting the two-phase pressure drop of refrigerant-oil mixture in circular metal-foam tubes was developed, and it agrees with 90% of the experimental data within a deviation of ±20%.

Influence of metal foam on heat transfer characteristics of refrigerant-oil mixture flow boiling inside circular tubes

The influence of metal foam on heat transfer characteristics of refrigerant and oil mixture flow boiling inside circular tubes was experimentally investigated on two horizontal circular tubes filled with 5 PPI and 10 PPI copper foams, respectively. The experimental conditions include saturation pressure of 995 kPa, mass flux from 10 to 30 kg m−2 s−1, heat flux from 3.1 to 9.3 kW m−2, inlet vapor quality from 0.175 to 0.775 and oil concentration from 0% to 5%. The experimental results reveal that metal foam enhances the flow boiling heat transfer coefficient of refrigerant by a maximum of 185%, while the enhancement effect of metal foam is weakened at oil presence conditions. A new correlation is developed, which can agree with 98% of the experimental data within a deviation of ±30%.

Heat Transfer and Friction Factors for Flows Inside Circular Tubes With Concavity Surfaces

Heat transfer and friction coefficients measurements have been obtained for fully developed, turbulent internal flows in circular tubes with six different concavity (dimple) surface array geometries. Two different concavity depths and three different concavity array densities were tested using tube bulk flow Reynolds numbers from 20,000 to 90,000. Liquid-crystal thermography was used to measure the temperature distributions on the outside of the concavity tubes. Using the average heat transfer coefficient for the fully developed region, the overall heat transfer enhancements are compared to baseline smooth tube results. Friction coefficients are also compared to values for a smooth tube. Dimple depths of 0.2–0.4 relative to the dimple surface diameter were used, with surface area densities ranging from 0.3 to 0.7. Dimple arrays were all in-line geometries. The results showed that heat transfer enhancements for dimpled internal surfaces of circular passages can reach factors of 2 or more when the relative dimple depth is greater than 0.3 and the dimple array density is about 0.5 or higher. The associated friction factor multipliers for such configurations are in the range of 4–6. The present study provides a first insight into the heat transfer and friction effects of various concavity arrays for turbulent flows.

An Exact Solution for Fully Developed Temperature Distribution in Laminar Steady Forced Convection Inside Circular Tubes with Uniform Wall Temperature

In this paper the fully developed temperature distribution in laminar steady forced convection inside circular tubes with uniform wall temperature has been obtained using an exact analytical procedure. Up to now, the procedure for obtaining the temperature profile was a trial and error method. Kakac, Shah, and Aung [1], have reported an unpublished paper that proposed the solution in the form of a power series (cited in [1]). Any of the above processes seems quite sufficient to find the Nusselt number. However, a strictly exact analytical procedure does not exist. The aim of the present paper is to fill this gap in the literature. A numerical solution is also reported here. To achieve this goal a finite difference backward scheme with the truncation error of O(0.001) is utilized. It is observed that the results agree perfectly with those found by analytical solution. The agreement between the results of these two methods - numerical and analytical - left no suspicion that our analytical solution is the correct answer of the problem published for the first time.

An algebraic solution for a 2-D partial differential energy equation with a Robin boundary condition generated from the simpler solution for a Dirichlet boundary condition

Parallel theoretical and numerical analyses have been conducted for the predic tion of the mean bulk temperatures of hot fluids flowing inside circular tubes. Heat exchange between an internal forced flow and an external, normal forced flow of a cold fluid occurs through the tube wall. The formal mathematical formulation of this physical problem is expressed in terms of a 2-D, partial differential equation of parabolic type with a Robin boundary condition at the tube wall. The aim of the paper is to critically examine the thermal response of the internal flows implementing two different mathematical models: a 2-D differential-based model and a 1-D lumped model. The key element for the latter is that streamwise-mean, internal Nusselt numbers and circumferential-mean, external Nusselt numbers are taken from standard correlations equations based on a Dirichlet boundary condition at the tube wall. The combination of these Nusselt numbers leads to the calculation of a mean, equivalent Nusselt number, which serves to regulate the thermal interaction between the two perpendicular, unmixed fluid streams, one hot and the other cold. The computed results consistently demonstrate that the 1-D lumped model, associated with hand calculations of an algebraic expression, provides accurate estimates of the mean bulk temperature distribution when compared with the exact ones computed with the mathematical 2-D distributed model and a personal computer.

Development of a Flow Boiling Map for Subcooled and Saturated Flow Boiling of Different Fluids Inside Circular Tubes

The thermal behavior of a flow boiling system is represented by a flow boiling map to illustrate visually the relationships among various system parameters. An earlier flow boiling map by Collier (1981) does not include the effect of mass flux and is specific to water at low pressures. For other fluids, significant departures from the parametric trends displayed in Collier’s map have been reported in the literature (e.g., Kandlikar, 1988b). In the present paper, a new flow boiling map is developed to depict the relationships among the heat transfer coefficient, quality, heat flux, and mass flux for different fluids in the subcooled and the saturated flow boiling regions. The trends observed in the experimental data and correlations for water and refrigerants are used in deriving the present map. The particular areas where further investigation is needed to validate the trends are also indicated. In the subcooled boiling region, hTP/hlo is plotted against x with Bo as a parameter, while in the saturated boiling region, hTP/hlo is plotted against x with ρl/ρg and a modified boiling number Bo* as parameters. It is hoped that the map would prove to be helpful in explaining the role of different heat transfer mechanisms in flow boiling of different fluids.

Laminar forced convection to non-Newtonian fluids in ducts with prescribed wall heat flux

Heat transfer in laminar forced convection to non-Newtonian fluids in thermally developing flow inside circular tubes and parallel plate ducts with prescribed wall heat flux is solved exactly. The local and average Nusselt numbers are determined and accuracy of the existing approximate solutions is examined.

A simple stochastic model of two phase flow pressure drop accompanied by boiling inside circular tubes with in-line static mixers

A simple stochastic model has been developed for boiling pressure drop inside a circular tube with in-line static mixers. This model gives rise to the dimensionless correlating equation of the form: [f] = a[Pr m] −1 2 [Re L] −1 μm μ L ρ ρ −1 2 H se + xH LG C pmΔT −1 2 This correlation shows good agreement with the experimental data.

Discussion: “Transient Freezing of Liquids in Forced Flow Inside Circular Tubes” (O¨zis¸ik, M. N., and Mulligan, J. C., 1969, ASME J. Heat Transfer, 91, pp. 385–389)

Discussion: “Transient Freezing of Liquids in Forced Flow Inside Circular Tubes” (O¨zis¸ik, M. N., and Mulligan, J. C., 1969, ASME J. Heat Transfer, 91, pp. 385–389)

Transient Freezing of Liquids in Forced Flow Inside Circular Tubes

The transient freezing of a liquid flowing inside a circular tube is investigated analytically under the assumption of a constant tube wall temperature which is lower than the freezing temperature, constant properties, a slug-flow velocity profile and quasisteady state heat conduction in the solid phase. The variation of the local heat flux and the profile of the solid-liquid interface during freezing has been determined as a function of time and position along the tube. The analysis produced steadystate heat transfer rates and profiles for the solid-liquid interface which agreed well with experiments.

Closure to “Discussion of ‘Transient Freezing of Liquids in Forced Flow Inside Circular Tubes’” (1969, ASME J. Heat Transfer, 91, p. 389)

Closure to “Discussion of ‘Transient Freezing of Liquids in Forced Flow Inside Circular Tubes’” (1969, ASME J. Heat Transfer, 91, p. 389)