Influence of the developing region of the thermal boundary layer on heat transfer during vapor condensation on horizontal tube bundles

The purpose of this publication is to describe a phenomenon that is fundamentally important and, at the same time, hardly elucidated in the literature. In the process of film condensation of vapor on the bundles of horizontal tubes, a developing region of the thermal boundary layer is formed on each tube of the bundle; the role of this region in heat transfer is important, and in some cases decisive. The paper presents experimental results on the influence of various contributions of the developing region on heat transfer during film vapor condensation on tube bundles. Based on the data obtained, an algorithm for calculating a condenser during condensation of a stationary vapor without non-condensable impurities is proposed

Combining Microstructured Surface and Mesh Covering for Heat Transfer Enhancement in Falling Films of Refrigerant Mixture

The article presents the experimental results of combining a basic microstructure with partly closed pores and a mesh covering for heat transfer enhancement at the film flow of a refrigerant mixture. To reveal the effect of the combined structure, heat transfer on a microstructured surface without a covering as well as on a smooth surface with a mesh covering only has been studied. All experimental series were carried out using a binary mixture of R114 and R21 refrigerants. The mixture film flowed down the outer surface of a vertical cylinder in the undeveloped turbulence regime, when the film Reynolds number varied from 400 to 1300. It is shown that a microstructured surface with a fin pitch of 200 μm, fin height of 220 μm, and longitudinal knurling pitch of 160 μm, created by deformational cutting, demonstrates significant heat transfer enhancement: up to four times as compared to a smooth surface. However, adding a mesh covering with an aperture of 220 μm and a wire diameter of 100 μm reduces the intensification. The mesh covering overlaid on a smooth surface also does not provide heat transfer enhancement as compared to the smooth surface itself. The absence or even deterioration of heat transfer enhancement on surfaces with mesh covering can be primarily associated with the low thermal conductivity of the mesh material and shortcomings of the applied method of mesh mounting. The possibility of deteriorating vapor removal due to the incorrect selection of mesh covering parameters was also analyzed. The heat transfer coefficient values obtained for basic microstructured surfaces were compared with the dependencies available in the literature for predicting pool boiling heat transfer on microfinned surfaces

Regimes of falling liquid film flowing over the vertical cylinder at contact angles up to 90° and Reynold number 50

The paper presents 3D numerical modeling of spreading dynamics of R21 (mol. fraction: 0.9) and R114 refrigerant mixture film. We considered an outer flow along a round vertical cylinder at Reynolds numbers of 50 and contact angles of 10°, 30°, 50°, 70° and 90°. The simulation was performed in OpenFOAM software on the basis of the volume of fluid (VOF) method. We obtained that the contact angle has a key effect on the wetted area due to the change of liquid spreading modes over the cylinder. At that, we distinguished the following flow modes: the stable jet mode, the cascade jet mode, the jet-droplet mode and the drying mode. These modes are similar ones for horizontal tubes. In some flow modes over the vertical cylinder, we demonstrated the existence of the back liquid flow between jets, directed against gravity

On the almost sure growth rate of sums of lower negatively dependent nonnegative random variables

For a sequence of lower negatively dependent nonnegative random variables Xn,n[greater-or-equal, slanted]1 , conditions are provided under which almost surely where bn,n[greater-or-equal, slanted]1 is a nondecreasing sequence of positive constants. The results are new even when they are specialized to the case of nonnegative independent and identically distributed summands and bn=nr, n[greater-or-equal, slanted]1 where r>0.Sums of lower negatively dependent random variables Nonnegative random variables Sums of independent and identically distributed random variables Almost sure growth rate

An Efficient Supply Chain Management Model: The Analysis of Domestic and International Practices

Abstract— The study discusses the challenge of providing an effective supply chain management model that would meet modern requirements. The purpose of the study is to analyze the theoretical and practical aspects of developing business processes of supply chain management, as well as to substantiate an efficient business logistics management model. The survey revealed that supply chain management is carried out in a highly competitive environment while business owners are in very difficult conditions: on the one hand, customer requirements are being expanded and diversified, and on the other hand, global logistics market entry requires better service and price cuts. It has been concluded that it is feasible to outsource IT logistics processes due to the global tendency to complicate supply chain management, the introduction of information technologies, increasing customer requirements, and major business processes within the supply chain. This is reflected through an increase in investment / demand in the IT industry, an increase in the number of providers, an increase in equipment, a shortage of specialists, etc. The trend observed in our country is consistent with the ones of developed economies. An algorithm for the implementation of the supply chain formation mechanism has been developed