Plunging Circular Jets: Experimental Characterization of Dynamic Pressures Near the Stagnation Zone

Spillways are a requirement for dams’ safety, mainly preventing overtopping during floods. A common spillway solution involves plunging jets, which dissipate a considerable flow energy in the plunge pool. Energy dissipation has to occur in a controlled manner to avoid endangering the dam foundation and river slopes. Indeed, a scouring process in the downstream riverbed will inevitably develop until equilibrium is reached, otherwise a suitable pre-excavated or concrete lined plunge pool has to be provided. This paper focuses on experimental studies in which particular attention was paid to the dynamic pressures in the plunge pool floor at the vicinity of the jet stagnation zone sampled at 2.4 kHz. A rectangular experimental facility, 4.00 m long and 2.65 m wide, was used as plunge pool. Tests involved a vertical circular plunging jet with velocity ranging from 5 to 18 m/s and plunge pool depth ranging from 4.2 to 12.5 jet diameters. Differences in dynamic pressure measurements are highlighted between transducers located in the inner and outer regions of the jet diameter footprint. Several parameters characterizing the dynamic pressures evidence trends tied with the jet velocity that, to the authors’ knowledge, were not dealt in previous research. These can derive from the coupling effects of consequent recirculating motions and air entrainment in the limited-size plunge pool. Both effects, increasing with velocity, cause an reduction in the efficiency of the diffusing jet shear layer. This aspect deserves further investigation to achieve a better understanding and more complete characterization.

Numerical analysis of a solar air heater with jet impingement - comparison of performance between jet designs

One of the ways to improve the performance of solar air heaters (SAH) is to use jet impingement on the absorber plate to cause turbulence mixing of air in contact with the plate and thereby augment the heat transfer coefficient. The objective of this work is to compare the thermohydraulic performance of a SAH with jet impingement through conical protruding jets and circular jets using finite element method based COMSOL Multi-physics software. The simulation studies were conducted for solar radiation in the range 500 – 1000 W/m2 and mass flow rate in the range 0.01 – 0.028 kg/s. The flow physics of the jet impingement process is investigated to understand the heat transfer and fluid flow behavior of the SAH with the chosen jet designs thereby obtain their performance insights. The outlet hot air temperature from the heater and its thermal efficiency are compared for different mass flow rates and solar radiations. Also the temperature distributions in the jet plate with the jet configurations are captured and their heat transfer characteristics compared to understand the thermo-fluidic behavior of the SAH. The results demonstrate improved performance of the novel conical protruded jet design that enhances the thermal efficiency up to 78.52%, which is an increase of 13.53% compared to the circular jet design. More elongated streamlines and higher turbulent kinetic energy with increased mass flow rate leading to a wide jet affected area inside the duct are the main reasons of its improved performance.

Effect of orifice spacing on twin circular parallel compressible jets

The interaction of Mach 0.5, 0.8, and 1.0, parallel, twin circular jets issuing from orifices with center-to-center spacing S/D, where S is the center-to-center distance and D is orifice diameter, 2, 4 and 6 has been investigated experimentally. The characteristics of twinjets are analyzed based on the centerline Mach number decay, exit Mach number and ratio of orifice spacing. As the spacing between the orifice increases, the maximum Mach number point of the combined jet moves downstream. For the Mach numbers studied it is found that as the S/D increases the effect of the counter-rotating vortices on jet mixing decreases. The rate of the twinjet interaction also decreases with S/D increase. As the jets propagate downstream their center-to-center distance decreases continuously and the jets merge to become single jet, for all S/D studied.

Coaxial Circular Jets—A Review

This review article focuses on the near-field flow characteristics of coaxial circular jets that, despite their common usage in combustion processes, are still not well understood. In particular, changes in outer to inner jet velocity ratios, ru, absolute jet exit velocities and the nozzle dimensions and geometry have a profound effect on the near-field flow that is characterized by shear as well as wake instabilities. This review starts by presenting the set of equations governing the flow field and, in particular, the importance of the Reynolds stress distributions on the static pressure distribution is emphasized. Next, the literature that has led to the current stage of knowledge on coaxial jet flows is presented. Based on this literature review, several regions in the near-field (based on ru) are identified in which the inner mixing layer is either governed by shear or wake instabilities. The latter become dominant when ru≈1. For coaxial jets issued into a quiescent surrounding, shear instabilities of the annular (outer) jet are always present and ultimately govern the flow field in the far-field. We briefly discuss the effect of nozzle geometry by comparing the flow field in studies that used a blockage disk to those that employed thick inner nozzle lip thickness. Similarities and differences are discussed. While impinging coaxial jets have not been investigated much, we argue in this review that the rich flow dynamics in the near-field of the coaxial jet might be put to an advantage in fine-tuning coaxial jets impinging onto surfaces for specific heat and mass transfer applications. Several open questions are discussed at the end of this review.

Axis-Switching Behavior of Liquid Jets Issued from Non-Circular Nozzles Under Low-Intermediate Pressure

HighlightsThe flow behavior of water jets discharged from different orifices was investigated.High-speed photography (HSP) was used to obtain surface structures and spread characteristics of water jets.The deformation process in axis switching related to the corner vortices effect of non-circular jets was researched by numerical simulation.The axis switching of non-circular jets enhances entrainment ability of the jet.ABSTRACT. Low-intermediate pressure sprinkler irrigation systems are important research topics in the field of water-saving irrigation. Non-circular nozzles improve spray uniformity at lower pressures and are key components of sprinkler irrigation systems. In this article, the behavior of discharged water jets from nozzles with circular, square, and equilateral triangular orifices designed with the same flow rate was investigated. High-speed photography (HSP) was used to capture jet characteristics in the near field (z<20D). The largest spread angle was obtained for the square jet, which was on average 37% larger than that of the circular jet. In addition, numerical simulations were performed to analyze the axis-switching process using the large-eddy simulation (LES) method and the coupled level-set and volume of fluid (CLSVOF) method. The axis-switching phenomenon was observed in non-circular jets, in which surrounding air mixed with the jet and promoted the formation of thin diaphragm structures. The deformation process that occurs in axis switching is described according to the simulated vorticity and velocity fields. The research results suggest the axis-switching phenomenon is induced by corner vortex motions produced by the polygonal orifice, which accelerate the decay of the axial velocity and increase the jet entrainment rate. Thus, the effect of corner vortices should be considered in the design of polygonal nozzles. Keywords: Axis switching, High-speed photography, Liquid jet, Low-intermediate pressure sprinkler irrigation, Non-circular nozzle, Numerical simulation.

Heat Transfer and Flow Visualization of Equilaterally Staggered Jet Arrangement on a Flat Surface

The present study discusses two equilaterally staggered jet arrangements for uniform cooling of flat surface. The equilateral staggered arrangement consists of a circular jet surrounded by four neighboring chamfered jets at an angle of 45° called as chamfered configuration. An equilateral staggered jet arrangement consisting only of circular jets is considered as the non-chamfered configuration. Large-eddy simulations and Mie-scattering imaging techniques are discussed for heat transfer and flow visualization of equilateral staggered jet configurations. Formation of number of eddies characterizes the flow feature. The turbulence quantities of the jet configurations determine the amount of heat transfer. The eddies are formed due to recirculation which later breaks into smaller parts by the incoming jet fluid. The flow features basically constitute of jet to jet interaction, jet interference and upwash flow. It is also noticed that every jet cools an independent area. However with the inclusion of chamfered jets the region of highest heat transfer shifts away from the jet centerline. This happens because of change in direction of flow of jet due to chamfering. The heat transfer results are discussed in terms of Nusselt number and temperature contours. A maximum variation of 22.8% in average Nusselt number is obtained between both the configurations while varying the gap ratios between 3 to 7. An increase of jet-to-jet spacing ratio from 2 to 7 shows improvement in heat transfer by 15.1% and 13.2% for non-chamfered and chamfered configurations respectively.