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Experimental results on the near field development of a turbulent rectangular wall jet with aspect ratio 10 that issues from a sharp-edged orifice at Re h ∼ 23,000 are presented and discussed, in comparison with results obtained in a free jet with identical initial conditions. Hot wire X-probe measurements on cross plane grids provide information on the 3D characteristics of the flow field. This work, besides presenting the main features of the jet, focuses on the effect of vorticity on the development of specific flow field characteristics. Mean vorticity components were estimated by interpolation and derivation from the mean and turbulent velocity measurements and the symmetries of the flow field were imposed by suitable averaging. Several terms of the axial vorticity equation are presented and discussed to uncover some complex flow physics, related e.g. to axis switching and the formation of a dumbbell shape of the jet outline, in the early stages of development.

This paper presents the mean and turbulent properties of a low Reynolds number round jet (Re 5500), with a fully turbulent pipe flow initial velocity profile, from nozzle exit to the establishment of self similarity. Experimental data acquired with a 2-D Laser Doppler Anemometry system include jet flow parameters and velocity statistics, such as spreading, entrainment, axial and radial distributions of the axial velocity and higher order moments. A “preserving core” region is identified, in analogy to the potential core region observed in jets with a uniform central initial profile. A transition region is following and thereafter, self similarity is approached in steps with the mean axial velocity attaining first self similar profiles, while the axial fluctuations, the radial fluctuations and the turbulent Reynolds stresses follow. The form of all the measured self similar profiles compares well to previous findings, although the values of the turbulent properties are always lower compared to those of contraction round jets of significantly higher Reynolds number. The triple velocity products and skewness and flatness factors indicate a different eddy structure of the present jet compared to that of earlier reported contraction jets.

In this study, the potential of near infrared heating (NIR) is assessed experimentally in comparison to forced air heating. To this end the night time heating needs for two identical, scaled down, experimental greenhouses located in central Greece were monitored for one thermal period. Four IR electric lamps were used to heat the first greenhouse whereas the second one was equipped with an electric forced air heater and an electric fan which was used for the homogenization of the temperature distribution. The output of several thermocouples located at the canopy area, on the cover and in the greenhouse air in both greenhouses were recorded every minute and 10 minutes averages were stored on a data logger, along with parameters characterizing the outdoors macroclimate, including outside and sky temperatures, wind speed and relative humidity measured by a suitable meteorological mast. The two heating systems were operated automatically to keep a reference temperature at the canopy within certain limits during nights (14-16 °C), suitable for the growth of lettuce, which was used as the test cultivation. Both heating systems maintained successfully the required conditions at the canopy for the growth of the plants. The energy consumption was recorded based on the time of operation of the heating systems. The internal air temperature in the IR heated greenhouse was always several degrees lower than the reference temperature of the plants resulting in significant energy consumption savings for the IR heated greenhouse. Savings of 38-50% were measured for the IR system in relation to the consumption of the conventionally heated greenhouse during the thermal period summing up to a total mean of 43%.

An experimental study of the thermal field established, over a horizontal heated plate (1000 × 500 × 12 mm 3 ) in an open top cavity, of aspect ratio Γ = 2 has been conducted by means of local temperature measurements as a function of distance from the wall. The working fluid was air at atmospheric pressure and the imposed temperature conditions correspond to Rayleigh numbers (Ra) of order 10 8 . Temperature measurements were accomplished on the centerline over the plate using a fine thermocouple ( d = 0.025 mm). The thermal boundary layer thickness was estimated close to 2.0 mm. Distributions of mean and rms temperatures are presented and discussed along with the corresponding skewness and flatness distributions. The dependence of skewness and flatness factors of the overall probability density function (PDF) of an intermittent random process is illustrated. Based on these results the form of the temperature PDFs over the plate is discussed. It is shown that the exponential form of the distributions observed in the present experiments, as in several experiments in hard turbulence, is compatible with the intermittent presence of fluid structures with different origin and past history, which did not have the time to become homogenized. Power spectra density results at the same locations indicate the presence of a characteristic angular frequency ω p , whereas secondary peaks observed at higher frequencies provide further support to the concept of an intermittent process and the presence of fluid structures with different past history.

Joule’s first-law dictates that when electric current passes through a conductor heat is generated. The scope of this work is to investigate the Joule heating effect of two types of carbon fiber tows in dry form. The tows are used as primary heating elements on various preform structures that require accurate and uniform temperature control, using a DC power supply. The investigated temperatures, ranged from 300 K up to 650 K. A finite difference scheme was developed, in order to predict and quantify the transient phenomenon and is verified against experimental results. In order to exploit in-depth the phenomenon and achieve very good agreement between numerical and experimental results, temperature dependence of thermal and electrical properties (specific heat capacity, total hemispherical emissivity, and volume resistivity) was examined under controlled vacuum environment, ensuring no fiber oxidation.

The development of the turbulent flow field generated due to the interaction of grid turbulence with a swarm of bubbles is investigated experimentally, in a vertical channel of rectangular cross section. Void fraction and streamwise mean and rms velocity distributions have been measured at several distances from the grid, with an optical probe and Laser Doppler Velocimetry, in relation to the air flow rate ratio. The obtained results indicate that close to the grid the void fraction and velocity distributions are dictated by the bubble injectors’ location on the grid. Downstream the void fraction distribution changes to a double peak pattern. The velocity distribution is characterized by a shear layer between the wall area and the central area of the channel. The extend of this shear layer is increasing as the distance from the grid and the gas flow rate ratio are increasing, and is associated with a corresponding increase of the turbulence fluctuations. Autocorrelation and spectra measurements at the centre of the channel show a reduction of the flow scales for low void fraction. Consistently, power spectra distributions indicate that bubbles cause a redistribution of energy manifested by the relative enhancement of the intermediate scales’ energy content and a consequent reduction in the larger scales. These trends are gradually alleviated and reversed at large distances from the grid, as the air flow rate is increased.

The effects of different boundary conditions on the near field development of a turbulent rectangular jet issuing from a smooth contraction nozzle of aspect ratio 15 have been studied experimentally with x -wire Hot-Wire Anemometry (HWA). Four different configurations resulting from the presence or absence of endplates–configuring a jet out of a wall–and sidewalls, confining the jet at the planes of the short side walls of the exit nozzle, under identical inlet conditions have been considered. The mean streamwise and lateral velocity and associated turbulent characteristics up to an axial distance of 35 nozzle widths, H , have been measured. Results comprise centreline data for three Reynolds numbers, based on mean exit velocity and nozzle width, Re = 10,000 , 20,000 and 30,000 as well as lateral distributions at 13 downstream locations for Re = 20,000 . In the absence of sidewalls the presence or absence of an endplate has little effect on the development of the flow field. Measurements made in the presence of sidewalls indicate that the presence of the endplate results in a jet with higher lateral outward velocities within the jet’s shear layers and smaller inward lateral velocities outside the jet’s edges, followed by higher turbulent intensities of both streamwise and lateral components in the early stages of development. Furthermore, in configurations comprising sidewalls, the presence of an endplate results in a faster spreading and decaying jet whereas a reduction of the momentum flow in the far field is also observed.

Parameters that affect the temperature at which service hot water (SHW) is offered by an immersed tube heat exchanger (HX), inside a flat plate Integrated Collector Storage Solar Water Heater (ICSSWH), are examined numerically, by means of Computational Fluid Dynamics (CFD) analysis. The storage water is not refreshed and serves for heat accumulation. Service hot water is drawn off indirectly, through an immersed serpentine heat exchanger. For the intensification of the heat transfer process, the storage water is agitated by recirculation through a pump, which goes on only when service water flows inside the heat exchanger. Three main factors, which influence the performance, are optimized: The position of the HX relative to tank walls, the HX length and the tube diameter. All three factors are explored so that to maximize the service water outlet temperature. The settling time of the optimum configuration is also computed. Various 3-D CFD models were developed using the FLUENT package. The heat transfer rate between the two circuits of the optimum configuration is maintained at high levels, leading to service water outlet temperatures by 1–7 °C lower than tank water temperatures, for the examined SHW flow rates. The settling time is retained at sufficient law values, such as 20 s. The optimal position was found to lay the HX in contact with the front and back walls of the tank, with an optimum inner tube diameter of 16 mm, while an acceptable HX length was found to be about 21.5 m.

An experimental study is presented regarding the influence of sidewalls on the turbulent free jet flow issuing from a smoothly contracting rectangular nozzle of aspect ratio 15. “Sidewalls” are two parallel plates, flush with each of the slots’ short sides, practically establishing bounding walls extending the nozzle sidewalls in the downstream direction. Measurements of the streamwise and lateral velocity mean and turbulent characteristics have been accomplished, with an x -sensor hot wire anemometer, up to an axial distance of 35 nozzle widths, for jets with identical inlet conditions with and without sidewalls. Centreline measurements for both configurations have been collected for three Reynolds numbers, Re D = 10,000, 20,000 and 30,000. For Re D = 20,000 measurements in the transverse direction were collected at 13 different downstream locations in the range, x = 0–35 nozzle widths, and in the spanwise direction at three different downstream locations, x = 2, 6 and 25 nozzle widths. Results indicate that, the two jet configurations (with and without sidewalls) produce statistically different flow fields. Sidewalls do not lead to the production of a 2D flow field as undulations in the spanwise mean velocity distribution indicate. They do increase the two-dimensionality of the jet increasing the longevity of 2D spanwise rollers structures formed in the initial stages of entrainment, which are responsible for the convection of longitudinal momentum towards the outer field, establishing larger streamwise mean velocities at the jet edges. In the near field, up to 25 nozzle widths, lower outward lateral velocities in the presence of the sidewalls are held responsible for the decrease of turbulent terms including rms of velocity fluctuations and Reynolds stresses. Skewness factors increase monotonically across the shear layers from negative values to positive forming sharp peaks at the outer edges of the jet, illustrative of the presence of well defined 2D roller structures in the jet with sidewalls.

The recirculating flow field generated by a swirling jet and a coaxial annular stream entering a pipe is investigated with the use of 2D-DPIV. Parametric change of inlet flow rates (constant tangential injection with change of annular flow and vice versa) is being considered in order to study the mean and turbulent flow field. A recirculation bubble stabilized close to the swirler exit is the dominating feature of the interaction between the inner swirling jet and the annular stream. Results are discussed in terms of bubble topology and dynamics on the basis of a modified Rossby number that appears to describe the trends of the complex flow field.

The influence of a secondary, parallel, low Reynolds number, round jet on the development of a turbulent axisymmetric jet with Re 5500 is studied experimentally by means of 2 D laser Doppler anemometry. Two weakly turbulent jets of Reynolds numbers, 1200 and 1800, are considered. In both test cases the secondary jet is fully absorbed by the primary one. The mean and turbulent flow structure of the standalone primary jet is compared to previous studies referring to the jet’s development before and inside the self-similarity region. The flow field of interacting jets is then analyzed and discussed. Characteristic profiles are presented at locations within the merging and the mixing zones. In the early stages of development, the patterns of both jets can be identified. Within the merging region, besides the absorption of the secondary jet, the measurements indicate a spatial suppression of primary jet’s characteristics. Further downstream, the profiles resemble to those of a standalone jet. Higher values of the mean and turbulent terms are observed while the profiles extend over a wider region, in accordance with the secondary jet’s Reynolds number. Results of the present study demonstrate that skewness and flatness factors can be used as indicators of small scale mixing.

The design and operational characteristics of a 12-sensor hot wire probe for three-dimensional velocity–vorticity measurements in turbulent flow fields is described and discussed. The performance of the probe is investigated in comparison with X-sensor probe measurements in the near field of a rectangular turbulent jet with aspect ratio 6. Measurements have been conducted at Reynolds number Re D = 21,000 at nozzle distances of x/D = 1, 3, 6 and 11, where D is the width of the nozzle. The results obtained with the 12-sensor probe compare well to the results of the X-sensor probe. Distributions of mean and fluctuating velocity–vorticity fields are presented and discussed. Among the results the most prominent is the experimental confirmation of the high levels of fluctuating vorticity in the shear layers.

In the present paper, an experimental investigation of the humidifying and desulfuration of flue gas by water spraying is conducted in order to increase the ability of the self-desulfuration efficiency of Ca composition in coal in a power plant boiler. The experimental facility, nozzle structure, measurement method and instruments are introduced. The influence of nozzle structure, the way to enter the liquid and the ratio between phases on the spray characteristics including size distribution of the droplet and spray angle is studied. The results show significant variation and show how to optimize the design and operation of the nozzles.

Infrared (IR) heating as alternative to forced air heating has been studied experimentally for 70 days in two identical, small-scale, experimental greenhouses. The two heating options were implemented with four IR electric lamps and an electric forced air heater correspondingly. The microclimate in the greenhouses was monitored with thermocouples. Parameters characterizing the outdoors macroclimate were also monitored. Lettuce was used as the test crop, and the two heating systems were operated automatically to maintain a suitable reference temperature at the canopy. The investigation focused on night heating needs. Results indicated that the internal air temperature in the IR-heated greenhouse was always kept several degrees lower than the value targeted for the canopy. This lower temperature resulted in 43% average energy savings for the IR-heated greenhouse, compared to the conventionally heated one. A simple numerical model was used to estimate the potential energy savings in a production-scale greenhouse.

In the present work, the hemodynamic field of an occluded artery is studied experimentally. 2D D.P.I.V measurements are presented regarding the statistically mean and turbulent flow field created in an artery having a double stenosis with over 75% occlusion. In this manuscript, the steady state case is considered. Experimental results show the influence of the stenoses on the formation of recirculation zones and the effect of flow characteristics on the shear stresses developing in the artery model.

A new method is introduced for the analysis of LDA and PDA optical setups, especially suited for larger particles or bubbles. The properties of an optical setup are studied for a specific particle size in an effective control volume defined as the Locus of the Centres of the particles producing interference signals. The effect of the particle size on the size and position of such a control volume are evaluated and discussed. The convenient description using control volume fringes is adapted to suit larger particles and used to derive the equations of LDA and PDA measurements. The method sets new standards for the estimation of the active control volume as a function of the particle size, for dispersed two phase flows measurements.

It is well known that sensors measure the flow properties in a “control” volume rather than at a single point. This appears to be systematically ignored in void fraction measurements in dispersed two phase flows. In this work a correction factor is proposed to account for the mentioned inaccuracy in evaluating void fraction in two phase flows with a line sensor. The theory was developed to address the problem in the case of employing a hot film sensor in bubbly flows. In this derivation effects of deformation and deflection of the bubbles by the probe were not considered.

An experimental study of the thermal field established, over a horizontal heated plate (1000×500×12 mm3) in an open top cavity, of aspect ratio Γ = 2 is presented. The working fluid was air at atmospheric pressure and the imposed temperature conditions correspond to a Rayleigh number Ra = 4.1×108, based on the cavity height and the temperature difference across it. Schlieren visualization was used to identify characteristic structures developing over the heated plate. Local temperature measurements along the vertical axis over the plate centre were accomplished using a fine thermocouple (d = 0.025 mm). Distributions of mean and rms temperatures are presented along with the corresponding skewness and flatness distributions. The exponential form of the PDF temperature distributions observed in the present experiments, as in several experiments in hard turbulence, is compatible with the intermittent presence of fluid structures with different origin and past history. Temperature PDFs have been approximated by mixtures of weighed Gaussian distributions to illustrate this concept.

Local measurements of void fraction and continuous phase velocity field in water-air bubble, grid turbulence were conducted in a channel of vertical, square test section. The measured statistics indicate that, due mainly to the interaction of mean shear with the dispersed phase, the turbulence structure of the flow is modified. The observed change is characterized by a strong spatial dependence of void fraction and liquid flow properties, and the emergence of two spatial regions controlled by different physical processes. Intensity measurements indicate significant departure from isotropy in the flow. Two distinct regimes corresponding to low and high values of void fraction have been also identified. The autocorrelation and spectra measurements indicate that for low void fraction the scales of turbulence decrease while for higher values of void fraction increase again and inverse cascade is observed.

The potential advantages of night-time heating of greenhouses by modern infrared (IR) radiative sources, instead of forced hot air, are examined experimentally and theoretically. Measurements of indoor and outdoor conditions during typical cold nights in central Greece were taken in an experimental greenhouse using either forced hot air or IR heating. A simple theoretical model that contains all the essential physics was developed and subsequently used in parametric studies. Experimental and simulation results confirmed that, with IR heating, inside air temperatures several degrees lower than the desired plant canopy temperature were sustained, and that this temperature difference increased proportionally to the nightly drop in outside temperature. The model estimated energy savings in the order of 45% to 50% using the IR sources currently available, and predicted significant further benefits from improvements in the radiative efficiency of the IR sources.