Laboratory for Heat Exchange Control in Phase and Chemical Transformations

Scientific results:

For the first time with the use of optical methods we experimentally studied the turbulent structure of a separating flow behind a backward-facing step under the influence of rectangular or triangular tabs (barbs). It was determined that tabs significantly deform the field of both a mean flow and the distribution of pulsation values. An especially strong influence is observed in the near field of the flow behind the step, which improves the ventilation of this zone and here it will be possible to observe a strong intensification of heat exchange.

We have experimentally studied heat exchange intensification when vortex generators in the form of tabs of various shapes impact separating flows. For the first time we found a strong intensification of heat exchange directly in the near zone of a separating flow behind a backward-facing step. The effect of amplification of heat transfer intensity is caused by the destruction of the dead zone of the recirculation zone and the growth of the turbulence of the flow. We have demonstrated the possibility of practical use of tabs to intensify exchange processes as well as the level of unevenness of heat removal in the transverse direction.

Our researchers have developed a physico-mathematical model based on eddy-resolving methods for the numerical research of flow and heat exchange in the intercostal cell having various geometries, the prehistory of the flow and the variation of the Reynolds number. It has been demonstrated that the use of various methods of modeling and closure model leads to significantly different data on the structure of both the turbulent field and heat exchange. For instance, the use of the k-omega SST model leads to a decrease in the Nusselt number. The best agreement with experimental data was achieved using the v2f model. Eddy-resolving approaches more adequately reflect the structure of the flow.

We have developed a physico-mathematical model describing the dynamics of a two-phase turbulent flow and heat transfer in channels of complex shapes and in the presence of separation zones with flow recirculation. The model is based on the use of the Euler approximation for both phases in a mean flow and in a fluctuation flow and accounting for the heat and mass transfer processes on the surface of particles (drops). We created software that allows to pefform computational research of a wide range of scientific and engineering problems, including optimization problems for various energy devices with multiphase flows.

The Laboratory has researched the possibility of increasing the thermal conductivity coefficient of phase-changing materials when introducing single-wall carbon nanotubes into its composition. The introduction of «TUBALL» single-wall graphene nanotubes in in an amount of 0.5 mass per cent lead to a 24 per cent increase in the thermal conductivity coefficient of P2 graphene in the solid state.

Within the model of a reacting boundary layer we have proposed a method to analytically assess the efficiency of organizing an efficient heterogeneous chemical process on the surface of solid particles around which a flow of reagents flows. Using experimental data on the characteristics of the sedimentation of a heat-shield coating from the gaseous phase, the adequacy of the developed model has been demonstrated. We have mastered methods of the diagnostics and modeling of bounded swirling two-phase flows, including those in the presence of chemical reactions. The obtained results allowed to develop new approaches to organizing combined heat and mass transfer in vortex chambers with a rotating layer of disperse particles.

Using the molecular dynamics method, we have studied the heat conductivity of fluids in nanochannels. It has been demonstrated that in the general case the thermal conductivity of a fluid in nanochannels is always lower than in an unbounded volume, and this decrease is the higher the smaller the scale of the channel. For the first time it has been demonstrated that the viscosity of a gas, in the same manner as its heat conductivity, is anisotropic. In nanochannels fluid transport coefficients depend significantly on the interaction of its molecules with atoms of the channel walls. By changing the material of the channel walls it is possible to efficiently control the viscosity and heat conductivity of the medium. The indicated anisotropy of the viscosity is observed not only in nano- but also in microchannels.

It has been demonstrated that the heat conductivity of nanofluids with metallic particles, in the same way as for oxides, depends on the size of nanoparticles, and it grows with its increase. Moreover, exceeding the thermal conductivity coefficient of all the studied nanofluids is significantly higher than predicted by the Maxwell theory.

Implementation of research results:

New data of the experiment in destroying a separating flow using tabs can be used extensively to intensify heat and mass transfer in poorly ventilated recirculation zones of operating channels of energy devices for various purposes. It is important that the parameter of thermohydraulic efficiency of such vortex generators stays at a high value when varying the geometric and discharge characteristics in a wide range.

The data of a detailed comparative a analysis of the conductivity of various models of turbulence for adequately describing the characteristics of flow and heat transfer demonstrate that the best agreement with the experimental can be achieved by using the v2f model. Eddy-resolving approaches more adequately reflect the structure of a flow. These conclusions are undoubtedly of practical interest when setting new research objectives for the numerical analysis of separating flows and for intensifying heat transfer. Such a conclusion can be made for problems of two-phase flows in the presence of phase transitions on the surfaces of particles or drops.

The experimental data on the significant increase in the heat conductivity of phase-change materials by introducing carbon nanotubes indicate a high potential for the use of such technologies for increasing the efficiency of cooling in a wide range of energy technology processes.

New data on coefficients of microfluidic transfer in channels with small linear dimensions have an important applied significance. The data collected for the first time indicate a strong influence of both the composition of the nanofluid and the material of the walls on exchange processes. This data will serve as the basis for the creation of engineering methods for computing microchannel heat transfer devices for various applications.

Education and personnel occupational retraining:

More than 20 students, undergraduates and graduate students took part in the project. Many of them successfully defended their final qualification theses. A number of programs and courses have been developed and introduced into the educational process:

• “Modeling and design of micro- and nanosystems” (Siberian Federal University).
• “Modeling of heat transfer processes in micro- and nanochannels” (Novosibirsk State University of Architecture and Civil Engineering).
• “Modeling in Ansys Fluent” (Novosibirsk State Technical University).
• "Thermodynamics and Heat Transfer" (Novosibirsk State Technical University).
• "Mechanics" (Novosibirsk State University).
• “Technical thermodynamics” (Novosibirsk State Technical University) and others.

Much attention was paid to training highly qualified personnel. During the implementation of the project, 4 Habilitated Doctor of Science theses and 8 PhD theses were defended by team members.

Cooperation:

• Indian Institute of Technology (India)
• Tianjin University, Chinese National Engineering Research Center (China PR)
• Institute of Chemical Technologies (Bulgaria):
• Institute of Thermomechanics (Czech Republic)
• University of Brighton (United Kingdom)
• Dalian University (China PR)
• University of Bergamo (Italy)