Experiment And Mechanism Study On Drag-Reduction By Additives In Pipe Flow

In the building HVAC system, the pumps consume a large proportion of the total building energy, and the former energy saving research was mainly focused on the design of the distribution system and operation adjustments. Because the additive drag-reduction phenomenon, which can lead to great reduction of friction drag by adding a little high polymer or surfactant into the fluid, could not only significantly reduce the friction coefficient during the fluid distribution process, but also decrease the heat loss (or cold loss) along the distribution network due to the reduction of the heat transfer. Applying additive drag-reduction to HVAC systems could largely reduce the pump power in the distribution systems, meanwhile increase the capability of water carrying on the heat and as a result, the initial cost of the distribution network drops. So it is of great significance for energy conservation in HVAC field to investigate additive drag-reduction. This paper studies on the feasibility of applying the additive drag-reduction technology to practical engineering project in water system of HVAC.Based on the characteristics of water distribution system in HVAC, we design and set up an experiment-table for additive drag-reduction with controlled temperature, alternative pipe diameters and circulation mechanical degradation. The temperature is controlled from 10℃to 80℃and the Reynolds number alters in the range of 500~100,000 so as to simulate the majority of flow regimes in HVAC systems. We conduct drag-reduction experiments in circulation system convenient for researching the performance of withstanding mechanical degradation of different drag-reduction additives. Parts of the pipes are dismountable so that we could investigate the drag-reduction effect of different pipe diameters. Before starting the drag-reduction experiments we experimentize with water to calibrate inner diameter of the experiment pipes, using Hagen-Poiseuille law for laminar flow and Prandtl-Karman law for turbulence as standard.To explore the characteristics of drag-reduction solution, such as drag-reduction onset,concentration effect,temperature effect,diameter effect and performance of withstanding mechanical degradation, the drag-reduction experiments conducted in the PVC and copper pipes with PAM and CTAC solution are designed and implemented. The experiment results show that the maximum percentage of drag-reduction of 10℃,200ppm PAM solution flowing in copper pipes could be 65%,which indicates significant effect of drag-reduction, but degradation is observed under high temperature condition. Therefore, PAM drag-reduction additive can be potentially applied in the chilled water delivery systems. The experiment results also demonstrate that the maximum percentage of drag-reduction of 60℃,300ppm CTAC solution flowing in copper pipes could be 65% and meanwhile, the critical Reynolds number of CTAC solution increases with the increasing of the solution concentration, however, saturation behaviour is observed when the CTAC solution is under low-temperature condition. For instance, the critical Reynolds number of 100ppm CTAC solution is about 12000 while the critical Re number of 400,500 and 600 ppm CTAC solution is almost a constant at 22500. Since it shows that CTAC has a certain characteristics of withstanding high temperature and mechanical degradation, its promising application in district cooling and under-floor heating systems is expected.Most of the pipe flows in the HVAC systems are turbulent flow, in which 20~30% of the energy loss is caused by the high order bifurcation of large scale vortex in turbulence central area while 70~80% energy loss is caused by turbulent bursting near boundary layer. Based on the research of turbulent coherent structure, we find that although the turbulent bursting is stochastic in space, it is a coherent structure with probable periodicity and determinacy structure, which could form an oscillation flow field in the boundary layer. So it is necessary to observe the dynamic behavior of macromolecule, viewed as microscale globose particles, in oscillation flow field when we research on mechanism of drag-reduction. This paper brings forward the conception of"oscillation viscosity of particle suspended fluid", constructs a mathematical physics model of the interaction between globose particles and oscillation flow field, and deduces the analytical expression of movement laws of microscale globose particles in oscillation flow field and oscillation viscosity of particle suspended fluid. This part of work is a good extension for the classic theory of viscosity of particle suspended fluid, meanwhile establishes a solid base for succeeding research on mechanism of drag-reduction in this paper.The results of this research demonstrate that the particles of drag-reduction additive will generate oscillation response when they come to boundary layer and effected by oscillation flow field caused by turbulent bursting. This process is able to lower the oscillation intensity of the flow field, and what is more important, the energy loss will result in an addition in dynamic oscillation viscosity in local field. The addition viscosity can obstruct formation of vortex tube and development of formed vortex tube to reduce the frequency and intensity of turbulent bursting, and restrain the turbulent bursting which is of significant contribution in turbulence friction, finally achieves macroscopical drag-reduction effect. The influences on the viscosity of particle suspended fluid by size, density and frequency of the suspended particles could be calculated with the analytical expression of oscillation viscosity deduced in this paper. For instance, the dynamic oscillation viscosity of 10wppmPEO(WSR-301)solution at peak frequency of energy spectrum increases 7.14% compared with the pure water and the maximum macroscopical drag-reduction effect could be achieved at the same time. The local dynamic oscillation viscosity of particle suspended fluid is a function of space and time, the larger the value near the boundary layer, the more ability of restraining the turbulent bursting could be obtained, as a result, the more significant effect of drag-reduction can be achieved. This is the new explanation to mechanism of drag-reduction raised in this paper.