This study focuses on an experimental and numerical heat transfer investigation of an impingement jet array with a concave target plate that resembles a turbine blade's leading edge. The array ...consists of nine circular jets with a diameter of Djet and the target plate has a curvature radius of R=Djet. The target plate also features film cooling holes in a staggered configuration so that each jet is surrounded by three film cooling holes. A Reynolds number of 30,000 is tested, while the separation distance H is altered between H/Djet=2.7 and H/Djet=4. In addition the jets are exposed to a crossflow (cf). The experimental heat transfer data are obtained by using the transient Thermochromic Liquid Crystal (TLC) method. In the Computational Fluid Dynamics (CFD) study 3D steady state Reynolds-Averaged Navier-Stokes (RANS) simulations with the software package OpenFOAM and the kω−shear−stress−transport (SST) turbulence model are performed. The numerical results from OpenFOAM agree well with the experimental data and local flow phenomena are captured correctly. The crossflow decreases the stagnation point heat transfer as well as the overall heat transfer but homogenizes the local heat transfer distribution. A smaller separation distance enhances the crossflow effects and generally increases the heat transfer level.
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•The effect of woven spacers on heat transfer and friction in channels was studied.•Experiments were performed by Thermochromic Liquid Crystals and image processing.•Numerical ...simulations were performed both for steady and unsteady flow.•A good agreement was obtained for the Nusselt number and the friction coefficient.•Results are relevant for the optimum design of Membrane Distillation (MD) modules.
Models of woven-type spacer-filled channels were investigated by Computational Fluid Dynamics (CFD) and parallel experiments in order to characterize the performance of Membrane Distillation (MD) modules. The case of overlapped spacers was analysed in a companion paper.
Experiments were based on a non-intrusive technique using Thermochromic Liquid Crystals (TLC) and digital image processing, and provided the distribution of the local convective heat transfer coefficient on a thermally active wall. CFD simulations ranged from steady-state conditions to unsteady and early turbulent flow, covering a Reynolds number interval of great practical interest in real MD applications. A specific spacer aspect ratio (pitch-to-channel height ratio of 2) and two different spacer orientations with respect to the main flow (0° and 45°) were considered.
Among the existing studies on spacer-filled channels, this is one of the first dealing with woven spacers, and one of the very few in which local experimental and computational heat transfer results are compared. Results suggest a convenience in adopting the 45° orientation for applications that can be operated at very low Reynolds numbers, since convenience decreases as the Reynolds number increases.
An algorithm is presented for the solution of an inverse heat conduction problem: reconstruct the steady-state distribution of the convection heat transfer coefficient on one wall of a slab from the ...temperature distribution on a plane embedded in the slab, assuming that the thermal boundary conditions on the other walls are known. The algorithm is based on the finite volume discretization of the slab and on the construction and subsequent inversion of square matrices linking the distributions of wall temperature and wall heat flux to that of temperature at the internal plane. The algorithm is first applied to a simulated temperature distribution, for which it is assessed for accuracy and compared with a simpler method based on the assumption of one-dimensional heat conduction. It is then applied to real experimental temperature distributions obtained by digitally processing thermochromic liquid crystals images in spacer-filled channels simulating membrane distillation modules. Tolerance to noise is demonstrated in both cases provided an adequate amount of filtering is applied to the primitive, high-resolution temperature distribution.
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•An algorithm was developed to solve inverse conduction problems by matrix inversion.•The algorithm was applied to a slab with Robin boundary conditions on one wall.•Results were compared with a simplified method which assumes 1-D heat flux.•Tests were conducted for both simulated and experimental temperature distributions.•The algorithm is accurate and tolerant of noise if data are adequately filtered.
Channels with ribs/baffles are widely applied in thermal engineering applications such as heat exchangers, solar water/air heaters and gas turbine cooling. In the present work, inclined and ...transverse twisted-baffles (I-TBs/T-TBs) are developed for heat transfer enhancement in a channel. The effects of Reynolds number (Re = 4000, 8000, 12,000, 16,000 and 20,000) roughness pitch ratios (p/w = 4.0, 6.0, 8.0, 10.0 and 12.0) and baffle twist ratios (y/w = 2.0, 3.0, 4.0 and 5.0 corresponding to twisted-baffle loop numbers (N) of 5, 7, 8 and 9) were investigated using air (Prandtl number, Pr = 0.7) as a working fluid. Heat transfer behavior was examined using thermochromic liquid crystal temperature measurements. Typical transverse and inclined baffles were also tested for comparison. Heat transfer, flow friction and thermal performance are reported in terms of Nusselt numbers (Nu), Nusselt number ratios (Nu/Nus), friction factors (f), friction factor ratios (f/fs) and thermal performance indices (η). Experimental results reveal that under most conditions examined, I-TBs show better heat transfer, lower frictional losses and higher thermal performance than TBs, IBs and T-TBs. For I-TBs, maximum heat transfer and thermal performance are obtained at a moderate pitch ratio (p/w = 6.0). However, in cases of T-TBs, heat transfer and thermal performance monotonically increase with a decreasing pitch ratio. Friction losses caused by both I-TBs and T-TBs decrease considerably with increasing pitch ratio. For I-TBs, heat transfer and thermal performance monotonically increase with increasing twist ratios. However, for T-TBs, maximum heat transfer and thermal performance are obtained at y/w = 3.0. Over the present studied range, a channel with I-TBs having optimal geometry (p/w = 6.0 and y/w = 5.0) yields maximum thermal performance indices, as high as 1.98, which is greater than the maximum values yielded by channels with T-TBs, TBs, IBs, and smooth channels, by around 74.1%, 98%, 52.5% and 98.3%.
•Effect of channels with inclined twisted-baffles on thermal and heat transfer mechanisms is reported.•I-TBs having optimal geometry (p/w = 6.0 and y/w = 5.0) yields maximum thermal performance factors, as high as 1.98.•Transverse/Inclined twisted-baffles (T TB/I TB) can reduce the dead zones between baffles and increasing the thermal performance factor.
•Heat transfer in Rayleigh-Bénard convection can be strongly enhanced by surface roughness in the form of arrays of cubes.•The increased heat transfer is closely related to the ability of the thermal ...plumes to directly impinge on the opposing heat transfer plate.
This paper reports on an experimental study of the effects of surface roughness on the flow and heat transfer in cubical Rayleigh-Bénard convection cells for Rayleigh numbers between 107 and 1010. In the rough cells the top and bottom surfaces are equipped with square arrays of copper cubes. In line with other studies, three different regimes occur in the rough cells, with each regime having a different relation between the Nusselt number, Nu, and the Rayleigh number, Ra. In the first regime the Nu-Ra relation equals that of the smooth cell, but in the second and third regimes the Nu-Ra relation deviates from that of the smooth cell with significantly higher Nusselt numbers. To better understand these observations, the flow and temperature fields in both the smooth and rough cells were visualised by using particle image velocimetry with suspended thermochromic liquid crystals as flow tracer particles.
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•Effects of spacer orientation on heat transfer in MD channels was investigated.•Experiments were performed by a Thermochromic Liquid Crystals image analysis technique.•Predictions ...were obtained via direct numerical simulations and compared with experiments.•A good agreement was found both for local and global values of the Nusselt number.
The analysis of flow fields and heat or mass transfer phenomena is of great importance in the optimum design of spacer-filled channel geometries for a variety of membrane-based processes. In the present work, models of spacer-filled channels often adopted in Membrane Distillation are simultaneously investigated by experiments and Computational Fluid Dynamics (CFD). Experiments rely on a non-intrusive technique, based on the use of Thermochromic Liquid Crystals (TLC) and digital image processing, and provide the local distribution of the convective heat transfer coefficient on a thermally active wall. CFD relies on steady-state (laminar flow) simulations in the lower end of the Reynolds number range investigated and on direct numerical simulations in the upper end of this range. This latter is a region of great practical interest for real applications, in which the flow is chaotic but not fully turbulent, and neither steady-state simulations nor the use of turbulence models would provide satisfactory predictions. Results are reported and discussed for a specific spacer geometry (overlapped orthogonal cylindrical filaments) and different spacer orientations with respect to the main flow. To the authors’ knowledge, this is the first study in which an experimental validation of computational results concerning local heat or mass transfer is performed for spacer-filled channels.
•An experimental and numerical validation studies were conducted on jets impinging cooling inside a rotating semi-cylindrical channel.•These studies were conducted by considering a jet Reynolds ...number of 7,500 at five different rotation speeds (ranging from 0 to 200 rpm).•Numerical analysis was performed using the shear stress transport (SST) k–w turbulence model.•Two temperature measurement techniques, namely thermochromic liquid crystals (TLCs) and thermocouples, were adopted.•A good agreement was established between the numerical results and the experimental measurements.
A detailed experimental and numerical validation studies are conducted on internal channel jet impingement cooling where seven jets impinging inside a rotating semi-cylindrical channel. The study objective is to mimic the cooling flow structure in gas turbine leading edge by using the rotating the semi-circular channel. These studies are conducted by considering a jet Reynolds number of 7500 at five different rotation speeds (ranging from 0 to 200 rpm). Numerical analysis is performed using the shear stress transport (SST) k−ω turbulence model with a properly analyzed fine mesh containing around eight million nodes. A test setup with required instrumentation is developed inhouse for this study. Two temperature measurement techniques, namely thermochromic liquid crystals (TLCs) and thermocouples, are adopted. Further, the target surface temperature contours are precisely analyzed by comparing the TLC temperature measurements with the numerical temperature results. The captured temperature contours indicated points of minimum-temperature regions, which corresponded to the jet impingement regions. By examining the temperature distribution along the axial centerline, a good agreement has been established between the numerical results and the experimental measurements. For a jet Reynolds number of 7500, increasing channel rotation speed from 0 to 250 rpm has reduced the variation in temperature between different jets. The size of inlet port used to guide flow to the feeding duct has a strong impact on jet formation and flow structure, and it has led to different mass flow rate across jets. Furthermore, a small deviation between numerical and experimental data can be observed near the end side of the channel owing to the radial and lateral heat transfer losses and outlet flow restriction.
•Heat transfer in spacer-filled channels was experimentally investigated.•Thermochromic Liquid Crystals and Digital Image Processing were used.•A new test section allowed two-side heat transfer ...conditions to be achieved.•Several overlapped and woven spacer configurations were studied and compared.•A detailed sensitivity and uncertainty analysis was performed.
Heat transfer in spacer-filled plane channels was investigated by using thermochromic liquid crystals and digital image processing. A novel test section allowed the establishment of two-side cooling, closely representative of the conditions existing in real plane or spiral-wound membrane distillation (MD) modules. Several spacer configurations were investigated, differing in design (overlapped or woven filaments), pitch-to-height ratio and orientation with respect to the main flow direction. The Reynolds number Re ranged from 160 to 2500, as is typical in MD. At all flow rates investigated, symmetric two-side cooling provided significantly higher (25–35%) local and mean heat transfer coefficients hh than one-side cooling. The mean heat transfer coefficient increased when the pitch-to-height ratio increased from 2 to 4, and was much higher for woven than for overlapped spacers. Overlapped spacers with a flow attack angle ϕ of 0°-90° with respect to the two layers of spacer filaments exhibited significantly different distributions and mean values of hh on the two sides, the larger values occurring where the flow was orthogonal to the filaments. In all cases, the dependence of hh on Re could not be described by a simple power law.
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