Water-filled Enclosure Research Articles

Analysis of the effect of hot rotation cylinders on the enhancement of heat transfer in underfloor heating enclosures based on numerical and experimental results

Current heat transfer systems in the industry may result in problems such as low efficiency and environmental pollution. In this paper, the influence of heater power, speed and direction of rotation cylinders, and underfloor heating power are investigated on the Nusselt number in a water-filled enclosure with Pr=5.4 numerically and experimentally based on the Arduino control unit. The enclosure is under constant heat flux from the bottom wall. Using the mixed-Taguchi method and according to different considered cases, 36-state experiments were designed in the range of 2×108<Ra<8×108 and Re<4000. The problem was simulated by using the Realizable k-ε turbulence model, in 3D steady-state mode. Isotherms and streamlines were reported for each case and analysis revealed that the speed of rotation and the heater power has a more important role than other parameters on the heat transfer rate. In the best situation with a maximum speed of cylinders (S=S(max)), maximum floor heating system (qf˙=qf˙(max)), and maximum power of the heater (qr˙=qr˙(max)) the Nusselt number is 110.5 with an almost 22% increase in heat transfer rate. A polynomial surf fit for Nusselt number according to Re, Ra proposed and there are three parts to the variation range of Nusselt numbers based on Richardson numbers.

A Parallel Non-Dimensional Lattice Boltzmann Method for fluid flow and heat transfer with solid–liquid phase change

A new mesoscopic scale Parallel Non-Dimensional Lattice Boltzmann Method (P-NDLBM) is developed to dramatically speed up the computation of transient fluid flow and heat transfer problems. The P-NDLBM code using message passing interface (MPI) was compiled in Fortran. The model is presented in dimensionless form to simplify application to a broad range of problems. The effective domain decomposition, data transfer between CPUs, and various boundary conditions based on the P-NDLBM are presented. The code is validated by comparison to prior experimental and single CPU computational studies of cavity flow and the Rayleigh–Bénard problem. The time costs of simulations with equal mesh size are compared for CPU numbers from 1 to 64 to show the effectiveness of the approach. To illustrate the utility of the code, simulations of the transient temperature and fluid velocity during charging and discharging of an integral collector storage solar system are presented. The modeled system has encapsulated phase change embedded in the water-filled enclosure. The results illustrate the capability of the code to capture the phase transition within the encapsulated phase change material as well as the details of the overall flow structure and temperature field.

Numerical investigation of laminar and turbulent mixed convection in a shallow water-filled enclosure by various turbulence methods

In this study, we first modeled laminar mixed convection inside rectangular enclosure with moving wall and ‘aspect ratio’ = 10 and then the results were compared with other investigators. After showing verity of results, we continued our investigation with turbulent flow using standard , RNG and RSM models for Richardson numbers 0.1 to 10 and Ra = 6 × 109. The results showed that the turbulence intensity depends on the position. For example, in vertical walls and boundary layer, the flow was laminar and flow in center of enclosure was turbulent. In addition, the results indicated that as the Richardson number increased, the velocity changed happen more frequently in the vertical direction and fluctuations were seen more and more. Another conclusion to be drawn was that at natural convections, the Reynolds stresses curves gone under too many fluctuations all of which was the great impact of the ‘buoyancy force’ and the properties of ‘natural convection’ within this Richardson number. The last and foremost deduction was that at the ‘turbulence flow’, ‘heat transfer’ was generally greater than laminar flow and that was due to high level of mixing at the first. Key words: Reynolds stress, Richardson number, Boussinesq approximation, mixed convection heat transfer, turbulent intensity.

Natural Convective Flow and Heat Transfer in a Collector Storage with an Immersed Heat Exchanger: Numerical Study

A three-dimensional model and dimensionless scale analysis of the transient fluid dynamics and heat transfer in an inclined adiabatic water-filled enclosure with an immersed cylindrical cold sink is presented. The geometry represents an integral collector storage system with an immersed heat exchanger. The modeled enclosure has an aspect ratio of 6:1 and is inclined at 30deg to the horizontal. The heat exchanger is represented by a constant surface temperature horizontal cylinder positioned near the top of the enclosure. A scale analysis of the transient heat transfer process identifies four temporal periods: conduction, quasi-steady, fluctuating, and decay. It also provides general formulations for the transient Nusselt number, and volume-averaged water temperature in the enclosure. Insight to the transient fluid and thermal processes is provided by presentation of instantaneous flow streamlines and isotherm contours during each transient period. The flow field consists of two distinct zones. The zone above the cold sink is nearly stagnant. The larger zone below the sink is one of strong mixing and recirculation initiated by the cold plume formed in the boundary layer of the cylindrical sink. Correlations for the transient Nusselt number and the dimensionless volume-averaged tank temperature predicted from the model compare favorably to prior measured data. Fluid motion in the enclosure enhances heat transfer compared to that of a cylinder in an unbounded fluid.

Natural Convection from a Horizontal Tube Heat Exchanger Immersed in a Tilted Enclosure

Heat transfer rates of a single horizontal tube immersed in a water-filled enclosure tilted at 30 deg are measured. The results serve as a baseline case for a solar water heating system with a heat exchanger immersed in an integral collector storage. Experiments are conducted for isothermal and stratified enclosures with both adiabatic and uniform heat flux boundary conditions. Natural convection flow in the enclosure is interpreted from measured water temperature distributions. Formation of an appropriate temperature difference that drives natural convection is determined. Correlations for the overall heat transfer coefficient in terms of the Nusselt and Rayleigh numbers are reduced to the form NuD=0.675RaD0.25 for 9×105⩽RaD⩽4×107.

On cooling behavior of a vertical plate in a phase change material/water composite enclosure under pulsating heat load

This paper presents a numerical and experimental study concerning cooling characteristics of a pulsating heated vertical plate sandwiched between a substrate of phase change material (PCM) and an enclosure of water, forming a composite vertical rectangular enclosure. The vertical plate is assumed to have a uniform pulsating (on/off) volumetric heat source. The PCM considered in the present study is n-Octadecane. In the finite-difference simulation, the two-dimensional buoyancy-driven fluid flows developed in both the water-filled subenclosure and the molten PCM region of the PCM-filled subenclosure were modeled as laminar Newtonian fluid flow adhering to the Boussinesq approximation. Meanwhile, two-dimensional conduction is accounted for the plate heater as well as the solid PCM zone. Numerical results are presented to unveil the cooling behavior of the pulsating heat-generating plate through the PCM substrate and water-filled enclosure. Results of the parametric simulations reveal that the water layer has the better capability of heat dissipation than the PCM substrate. Heat dissipation from the plate through the PCM substrate is mainly via the latent heat absorption as associated with melting phenomenon. Moreover, numerical results obtained are compared with the corresponding experiments.

Experimental Investigation on Pulsating Horizontal Heating of a Water-Filled Enclosure

Experimental results of the natural convection generated by the time periodic horizontal heating of a square cross-section enclosure filled with water are reported. A pulsating (on/off) heat flux is delivered to the heating wall of the enclosure, with the opposite wall cooled by a high thermal capacitance system. All other surfaces are insulated. Heating periods from 32 to 1600 seconds and cycle-averaged heat-flux based Rayleigh numbers from 2.5 × 108 to 1.0 × 109 are considered. Results presented in terms of time series, phase-plane portraits, and cyclic evolution of surface-averaged cooling and heating wall temperatures illustrate the main characteristics of the evolution to periodic regime. Also presented are the cycle-averaged heat transfer coefficient versus heating period, and the corresponding average Nusselt number versus Rayleigh number for various heating frequencies. These results, which support published theoretical and numerical analysis, indicate that by tuning the heating period properly, the heat transfer across an enclosure can be enhanced. The results also reveal that short heating periods hinder the convection within the enclosure, in general (e.g., for Ra = 7.5 × 108 and a period of 32 s the heat transfer coefficient is 13 percent smaller than the steady heating value). The sensitivity of the transport phenomenon to pulsating heat is shown to depend strongly on Ra. Finally, a correlation for estimating the maximum heat transfer coefficient, derived from the experimental results, is presented.

Natural ventilation of an enclosure containing two buoyancy sources

This paper describes experiments and theoretical modelling of a naturally ventilated enclosure containing two point sources of buoyancy. Previous work on the flow and stratification that develop in a space owing to the presence of a single source of buoyancy has been extended to cover two sources of positive or negative buoyancy in an attempt to produce more realistic models of practical problems. Examples include naturally ventilated buildings, and industrial or medical processes involving dispersal of contaminants by air currents. The experimental technique involves the use of a water-filled enclosure with salt solution injected at two points. These sources of negative buoyancy form turbulent plumes within the enclosure. Two sources of unequal strength are found to produce a vertical density profile consisting of three distinct, fully mixed layers. The paper describes a theoretical model that successfully predicts the depths and densities of these layers. The positions of the interfaces between the three layers were found to be a function only of the effective area A* of the enclosure openings, the height of the enclosure H and the ratio of the strengths of the two sources of buoyancy B1/B2. The behaviour of a system with one source of positive buoyancy at the lower boundary and one of negative buoyancy at the upper boundary of the enclosure is also examined. This case has particular relevance to the prediction of thermal stratification in rooms with both cold and warm surfaces present. Again the height of the interfaces is dependent only on the geometry of the enclosure and the source strength ratio. Three distinct types of flow pattern are predicted and observed.

Conjugate natural-convection- conduction heat transfer in enclosures divided by horizontal fins

This article presents a numerical study concerning conjugate heat transfer across a vertical rectangular water-filled enclosure divided by multiple horizontal fins. The objective of the study is mainly to explore the effects of the conducting horizontal fins on heat transfer due to conjugate natural convection-conduction in the finned enclosure. Numerical results via a finite-difference method for the governing differential equations of the problem considered have been obtained for two different conductive fins with Ra = 10 3 to 10 7, AR = 1,5,8, and 10, N=1 to4, t=0.00001 to 0.125. The results demonstrate that the feasibility and effectiveness of using horizontal fins to enhance heat transfer across a vertical rectangular enclosure depend strongly on the aspect ratio, the Rayleigh number, the thermal conductivity, and the number of fins in the enclosure.

Visualization of temperature fields of transient natural convection with maximum density effect in a water-filled enclosure by chiral nematic liquid crystals.

In this work, we examined the application of a new liquid crystal, a chiral nematic type, to water. It has been said that this liquid crystal is stable even in water. However, its usefulness has not been confirmed since its applications have been very few. Accordingly, visualization of the temperature field of transient natural convection heat transfer of water with maximum density effect was carried out as an example to confirm the practical applications

Natural convection in rectangular enclosures heated from below and cooled along one side

Steady natural convection in a square, water-filled enclosure heated from below and cooled on one vertical side is studied analytically and numerically. Expansions for small Rayleigh number are developed to order Ra 2. It is found that the first contribution to the convective heat transfer occurs at order Ra 2. Asymptotic expressions are found for the temperature and heat transfer near the flux singularity on the enclosure floor. Finite difference numerical simulations indicate that the present condition (heating from below) is quite distinct from the case of cooling from below and heating on one vertical wall. Convective heat transfer is shown to be most significant when slightly less than half of the lower surface is heated.

A comparison of experimental and predicted results for laminar natural convection in an enclosure

An experimental and numerical study of laminar natural convection in a water-filled enclosure with an aspect ratio of 2:1 was conducted. Two opposing vertical walls were held at different uniform temperatures. The remaining four walls of the enclosure were insulated. The Rayleigh number based upon the height of the enclosure was 6.4×10 8 while that based upon the distance between the two isothermal plates was 8×10 7. The mean vertical velocities were measured using a two-beam laser Doppler anemometer system with forward scatter. The mean temperature profiles were obtained using a small, 0.07 mm diameter, chromelalumel thermocouple probe. A multilevel-multigrid algorithm was used to predict the flow for the experimental conditions studied. The predicted and experimental results for the mean vertical velocity profiles were compared and found to be in excellent agreement.

Natural convection in a partially divided enclosure

This paper describes an experimental and analytical study of the phenomenon of heat transfer by natural convection in a rectangular enclosure fitted with an incomplete internal partition. The experiments were carried out in a water-filled enclosure with adiabatic horizontal walls and vertical walls maintained at different temperatures. Heat transfer measurements and flow visualization studies were conducted in the Rayleigh number range 10 9–10 10, for aperture ratios h/H = 1, 1/4, 1/8, 1/16, and 0, where h and H are the height of the internal opening (above the partition) and the height of the enclosure, respectively. It is demonstrated that the aperture ratio h/H has a strong effect on both the heat transfer rate and the flow pattern. The second part of the study consists of an asymptotic analysis of the same phenomenon, valid in the limit of vanishing Rayleigh numbers. The flow and temperature fields in this limit are reported graphically for H/L = 0.5, Pr = 0.71 and 0.3 < h/H < 0.7, where L and Pr are the enclosure length and the Prandtl number, respectively.