Transient performance of fluid machinery during transient operating periods,such as startup and stopping,has been drawn more attentions recently due to the growing engineering needs.In this paper,the transient behavior of a prototype centrifugal pump with an open impeller during rapid startup period is studied experimentally.The variations of the rotational speed,flow rate,head,and shaft power during rapid startup period are recorded in experiments at different discharge valve openings.In addition,the non-dimensional flow rate and head are also used to analyze the transient behavior.The research result shows that the rise characteristic of the rotational speed is not basically changed by working points,while mainly depends on the startup characteristics of the driving motor.Compared with the rapid rise of the rotational speed,the flow rate rises slowly in the initial stage of startup.Moreover,the flow rate lags behind the rotational speed to rise to final stable value,and the delay becomes more severe with the increase of the discharge valve opening.The shaft power impact phenomenon generally exists in the process of startup.The non-dimensional analysis shows that the non-dimensional head is very high at the very beginning of startup,and quickly falls to the minimum,then gradually rises to final stable value,while the non-dimensional flow rate always shows the rise tendency during whole startup period.In conclusion,it is found from the non-dimensional results that the quasi-steady analysis is unable to accurately assess the transient flow during startup period.
To reduce the influence of adverse flow conditions at the fan hub and improve fan aerodynamic performance,a modification of conventional axial fan blades with numerical and experimental investigation is presented.Hollow blade root is manufactured near the hub.The numerical and experimental results show that hollow blade root has some effect on the static performance.Static pressure of the modified fan is generally the same with that of the datum fan,while,the efficiency curve of the modified fan has a different trend with that of the datum fan.The highest efficiency of the modified fan is 10% greater than that of the datum fan.The orthogonal experimental results of fan noise show that hollow blade root is a feasible method of reducing fan noise,and the maximum value of noise reduction is about 2 dB.The factors affecting the noise reduction of hollow blade root are in the order of importance as follows: hollow blade margin,hollow blade height and hollow blade width.The much smoother pressure distribution of the modified fan than that of the datum fan is the main mechanism of noise reduction of hollow blade root.The research results will provide the proof of the parameter optimization and the structure design for high performance and low noise small axial fans.
