Experimental study of single-row chevron-jet impingement heat transfer on concave surfaces with different curvatures

To address the curvature effect on single-row chevron-nozzle jet impingement heat transfer on concave surface,a series of experiments are conducted in the present investigation.Four concave surfaces including one semi-cylindrical concave surface and three parabolic concave surfaces with different width-to-depth ratios are tested under three typical Reynolds numbers(Re=5000,10000 and 15000)and several dimensionless nozzle-to-surface distances ranging from 1 to 8.The results show that the concave curvature has a clear impact on chevron-nozzle jet impingement heat transfer,tightly dependent on jet Reynolds number and impinging distance.In general,the semicylindrical concave surface produces the highest longitudinally-averaged Nusselt number at the leading line of concave surface.Under a low jet Reynolds number,the parabolic concave surface with a highly curved curvature produces higher longitudinally-averaged Nusselt number at the leading line and more uniform longitudinally-averaged Nusselt number distribution along the curvilinear direction.However,the longitudinally-averaged Nusselt number at the leading line of concave surface is the lowest for the highly curved surface under a high jet Reynolds number and large impinging distance.In comparison with the round-nozzle,chevron nozzle plays a more significant role on improving jet impingement heat transfer at small impinging distances.

Empirical study on directionalmillimeter-wave propagation in vehicle-to-infrastructure communications between road and roadside

With the increased demand for unmanned driving technology and big-data transmission between vehicles,millimeter-wave(mmWave)technology,due to its characteristics of large bandwidth and low latency,is considered to be the key technology in future vehicular communication systems.Different from traditional cellular communication,the vehicular communication environment has the characteristics of long distance and high moving speed.However,the existing communication channel tests mostly select low-speed and small-range communication scenarios for testing.The test results are insufficient to provide good data support for the existing vehicular communication research;therefore,in this paper,we carry out a large number of channel measurements in mmWave vehicle-toinfrastructure(V2I)long-distance communication scenarios in the 41 GHz band.We study the received signal strength(RSS)in detail and find that the vibration features of RSS can be best modeled by the modified two-path model considering road roughness.Based on the obtained RSS,a novel close-in(CI)model considering the effect of the transmitter(TX)and receiver(RX)antenna heights(CI-TRH model)is developed.As for the channel characteristics,the distribution of the root-mean-square(RMS)delay spread is analyzed.We also extend the twosection exponential power delay profile(PDP)model to a more general form so that the distance-dependent features of the mmWave channel can be better modeled.Furthermore,the variation in both RMS delay spread and PDP shape parameters with TX-RX distance is analyzed.Analysis results show that TX and RX antenna heights have an effect on large-scale fading.Our modified two-path model,CI-TRH model,and two-section exponential PDP model are proved to be effective.