In this study, composite aerogels with excellent mechanical properties were prepared by using carboxymethyl cellulose (CMC) as raw materials, with carboxylic carbon nanotubes (CNTs) as reinforcement. By controlling the mass fraction of CNTs, composite aerogels with different CNTs were prepared, and the surface morphology, specific surface area, compressive modulus, density and adsorption capacities towards different oils were studied. Compared to the pure CMC aerogel, the specific surface areas of CMC/CNTs were decreased because of the agglomeration of CNTs. However, the densities of composite aerogels were lower than pure CMC aerogel. This is because the CNTs were first dispersed in water and then added to CMC solution. The results indicated that it was easy for the low CMC initial concentration to be converted to low density aerogel. The compressive modulus was increased from 0.3 MPa of pure CMC aerogel to 0.5 MPa of 5 wt % CMC/CNTs aerogel. Meanwhile, the prepared aerogels showed promising properties as the adsorption materials. Because of the high viscosity, liquid possesses strong adhesion to the pore wall, the adsorption capacity of the CMC aerogel to the liquid increases as the viscosity of the liquid increases.

Pressure fluctuations in intake manifold of Internal Combustion Engine introduce a pulsating boundary condition to the centrifugal compressor. In this study, the performance of centrifugal compressor exposed to pulsating backpressure is experimentally investigated. Unsteady performance and surge characteristic of the compressor at different pulsating conditions are analyzed. Results show that instantaneous performance of the compressor forms a hysteresis loop encapsulating steady performance. A correlation of compressor unsteadiness with pulse frequency, magnitude, and local slope of the characteristic curve is obtained based on measurements. Cycle-average performance of the compressor is notably lower than that at constant conditions. Specifically, the averaged peak efficiency at pulsating condition drops 3%~7%. Fast Fourier Transformation method is applied to study surge characteristics at pulsating conditions. Results manifest that compressor surge is postponed remarkably by pulsating backpressure at all conditions. As the compressor approaches to small mass flow rate, flow fluctuations decay quickly and then increase dramatically when the surge happens, forming the variation with 'V-shape' for frequency domains. This phenomenon is resulted from the influence of the slope of characteristic curve on the filling-empty and compressor stability. Smaller slope produces less filling-empty and hence smaller size of the loop, but reduces the stability of compressing system and initiates surge.

Centrifugal compressor is exposed to pulsating backpressure due to the movement of intake valves in internal combustion engine. The performance of compressor deviates from the steady performance map, which affects the matching between turbocharger and engine. The behavior of compressor system at pulsation conditions are investigated via an in-house developed 1D unsteady code validated by experimental results. The influence of pulse frequency, magnitude and compressor characteristic curve on the compressor transient responses, including filling-emptying effect and wave dynamics, are analyzed. Results show that the strength of wave dynamics grows stronger with the increasing of pulse frequency, while the strength of filling-emptying effect increases first then decreases. The rise of pulse magnitude results in an almost linearly increasing of filling-emptying effect, while it can hardly affect the wave dynamics. Furthermore, the influence of pulsation magnitude and frequency represents the influence of local pressure gradient, and a correlation as quadratic curve can be evaluated between the pressure gradient and compressor unsteadiness. On the other hand, the influence of operating point, including the average mass flow rate and the slope of characteristic curve, is confirmed to be evidently smaller, comparing to the influence of pulsation frequency and magnitude. This study is helpful to estimate the behavior of compressor and the discrepancy of performance when operating at unsteady environment or matched with engine.

The flow in intake manifolds of internal combustion engine becomes more unsteady because of engine downsizing which is achieved by reducing cylinder number and increasing the boosting. Turbocharger compressor is thus exposed to the enhanced pulsating backpressure. This paper studies responses of a centrifugal compressor to the pulsating backpressure via experimentally validated numerical method. Firstly, CFD model with the volute and all impeller passages is established and validated by experimental measurements. Then the unsteady three-dimensional simulation is conducted on a single passage imposed by the pulsating backpressure conditions which are obtained by 1-D unsteady simulation. Results show that the performance of the passage evidently deviates from steady performance. Hysteresis loops of the performance appear at pulsating backpressure conditions, which encapsulate the performance at steady conditions. Moreover, the unsteadiness of the impeller performance is enhanced as the mass flow rate reduces. The performance and flow structures of the impeller near stall for the pulsating case are more favorable than that at corresponding constant backpressure. Furthermore, flow structures at points with the same instant mass flow rate are also notably different when they are located at different strokes of the pulse. The flow in the impeller is determined by not only the instant boundary condition but also the evolution history of flow field. The dynamic stall which is analogue to the phenomenon on pitching airfoil happens in the compressor and delays the instability of the device when it is exposed to pulsating backpressure. This study provides useful insights in the influence of pulsating backpressure on compressor performance in actual engine situations, from which better turbo-engine matching might be benefited.