Constant Surface Temperature Conditions Research Articles

Hydrothermal performance analyses of an isothermal tube with punched twisted tape turbulator (PTT)

This study represents a pioneering investigation into the impact of a punched twisted tape turbulator on the hydrothermal parameters of a pipe operating under constant surface temperature conditions. The punched turbulator was specifically designed by incorporating cuts with diverse geometries, including semicircular, square, and triangular shapes, into the basic twisted tape turbulator. To evaluate the performance of the punched twisted tape, a comparative analysis was conducted with both a smooth tube and a tube equipped with a basic turbulator. The criterion for selecting the optimal cut geometry was the thermal enhancement factor. Subsequently, the influence of the cut area on the hydrothermal parameters was thoroughly elucidated. The enhancement factor was again utilized to identify the most optimal area. Based on the comprehensive evaluations, it was observed that the turbulator with a semicircular cut geometry and a diameter of 5 mm exhibited superior thermal performance compared to all other geometries. By employing this specific turbulator design, a remarkable enhancement in heat transfer and pressure drop within the heat exchanger was achieved, offering improvements of up to 1.63 and 3.65 times, respectively, when contrasted with the plain tube. The maximum enhancement factor value recorded during the investigation reached 1.09.

Düz Bir Plaka Üzerindeki Hidrodinamik ve Isıl Sınır Tabaka Akışının Sayısal Olarak İncelenmesi ve Geçiş Kontrolü

In this study, Computational Fluid Dynamics (CFD) calculations are performed with ANSYS-Fluent for an external flow over a flat plate under constant surface temperature conditions. By using an Active Flow Control (AFC) method, the flat-plate is heated to manipulate the transition region. Calculations are performed for a steady and turbulent flow at 15 m/s free-stream velocity. Local skin friction coefficient and local heat transfer coefficient distributions along the flat-plate are investigated for laminar and turbulent boundary layers at various constant surface temperatures. For laminar and turbulent flow boundary layer characteristics, theoretical correlations in the literature are used to verify the numerical results. Results show that theoretical correlations are highly consistent with CFD results only in the laminar and turbulent regions and it is also shown that transition can only be predicted by CFD simulations. On the other hand, heating as an AFC method is found to be useful in delaying transition regime over a flat plate.

Heat Transfer in Air Flow Past a Bottom Channel Wall-Attached Diamond-Shaped Baffle – Using a CFD Technique

A computational analysis has been conducted to investigate turbulent flow and convective thermal transfer characteristics in a two-dimensional horizontal rectangular section channel with a hot lower wall-mounted diamond-shaped baffle. The calculations are based on the finite volume method, by means of Commercial Computational Fluid Dynamics software FLUENT, standard k-epsilon turbulence model with QUICK numerical scheme, and the SIMPLE discretization algorithm has been applied. The fluid flow and heat transfer characteristics, i.e., dynamic pressure coefficient, stream function, mean, axial, and transverse velocities, turbulent viscosity, temperature field, skin friction coefficients, local and average Nusselt numbers, and thermal enhancement factor are presented for flow Reynolds numbers based on the aeraulic diameter of the computational domain ranging from 12,000 to 32,000 at constant surface temperature condition along the upper and lower walls. Effect of the diamond configuration of the insulated baffle is studied numerically and the data obtained from this same baffle model are also compared with that of the simple flat rectangular baffle under similar operating conditions. Over the range under investigation, the improvements are found to be around 3.962 and 29.820 times higher than the smooth air channel with no baffle for heat thermal transfer and skin friction factor, respectively. The maximum TEF is around 1.292 at the highest Reynolds number value, Re = 32,000.

Modeling and analysis of solar air channels with attachments of different shapes

PurposeThis paper aims to report the results of numerical analysis of turbulent fluid flow and forced-convection heat transfer in solar air channels with baffle-type attachments of various shapes. The effect of reconfiguring baffle geometry on the local and average heat transfer coefficients and pressure drop measurements in the whole domain investigated at constant surface temperature condition along the top and bottom channels’ walls is studied by comparing 15 forms of the baffle, which are simple (flat rectangular), triangular, trapezoidal, cascaded rectangular-triangular, diamond, arc, corrugated, +, S, V, double V (or W), Z, T, G and epsilon (or e)-shaped, with the Reynolds number changing from 12,000 to 32,000.Design/methodology/approachThe baffled channel flow model is controlled by the Reynolds-averaged Navier–Stokes equations, besides the k-epsilon (or k-e) turbulence model and the energy equation. The finite volume method, by means of commercial computational fluid dynamics software FLUENT is used in this research work.FindingsOver the range investigated, the Z-shaped baffle gives a higher thermal enhancement factor than with simple, triangular, trapezoidal, cascaded rectangular-triangular, diamond, arc, corrugated, +, S, V, W, T, G and e-shaped baffles by about 3.569-20.809; 3.696-20.127; 3.916-20.498; 1.834-12.154; 1.758-12.107; 7.272-23.333; 6.509-22.965; 8.917-26.463; 8.257-23.759; 5.513-18.960; 8.331-27.016; 7.520-26.592; 6.452-24.324; and 0.637-17.139 per cent, respectively. Thus, the baffle of Z-geometry is considered as the best modern model of obstacles to significantly improve the dynamic and thermal performance of the turbulent airflow within the solar channel.Originality/valueThis analysis reports an interesting strategy to enhance thermal transfer in solar air channels by use of attachments with various shapes

Nonlinear radiated MHD flow of nanoliquids due to a rotating disk with irregular heat source and heat flux condition

This research is made to visualize the nonlinear radiated flow of hydromagnetic nano-fluid induced due to rotation of the disk. The considered nano-fluid is a mixture of water and Ti6Al4V or AA7072 nano-particles. The various shapes of nanoparticles like lamina, column, sphere, tetrahedron and hexahedron are chosen in the analysis. The irregular heat source and nonlinear radiative terms are accounted in the law of energy. We used the heat flux condition instead of constant surface temperature condition. Heat flux condition is more relativistic and according to physical nature of the problem. The problem is made dimensionless with the help of suitable similarity constraints. The Runge-Kutta-Fehlberg scheme is adopted to find the numerical solutions of governing nonlinear ordinary differential systems. The solutions are plotted by considering the various values of emerging physical constraints. The effects of various shapes of nanoparticles are drawn and discussed.

Numerical Analysis of Thermal and Aerodynamic Fields in a Channel with Cascaded Baffles

A computational fluid dynamic analysis of thermal and aerodynamic fields for an incompressible steady-state flow of a Newtonian fluid through a two-dimensional horizontal rectangular section channel with upper and lower wall-attached, vertical, staggered, transverse, cascaded rectangular-triangular (CRT), solid-type baffles is carried out in the present paper using the Commercial, Computational Fluid Dynamics, software FLUENT. The flow model is governed by the Reynolds averaged Navier-Stokes (RANS) equations with the SST k-ω turbulence model and the energy equation. The finite volume method (FVM) with the SIMPLE-discretization algorithm is applied for the solution of the problem. The computations are carried out in the turbulent regime for different Reynolds numbers. In this study, thermo-aeraulic fields, dimensionless axial profiles of velocity, skin friction coefficients, local and average heat transfer coefficients, and thermal enhancement factor were investigated, at constant surface temperature condition along the heated upper wall of the channel, for all the geometry under investigation and chosen for various stations. The impact of the cascaded rectangular-triangular geometry of the baffle on the thermal and dynamic behavior of air is shown and this in comparing the data of this obstacle type with those of the simple flat rectangular-shaped baffle. This CFD analysis can be a real application in the field of heat exchangers, solar air collectors, and electronic equipments.

Experimental investigation of optimum thermal performance and pressure drop of water-based Al2O3, TiO2 and ZnO nanofluids flowing inside a circular microchannel

This paper presents thermal performance and pressure drop characteristics of water-based nanofluids flowing through a horizontal circular microchannel under the constant surface temperature condition, experimentally. Al2O3 (13 nm), TiO2 (10–25 nm) and ZnO (18 nm) nanoparticles with 0.5, 0.7 and 1.0% volume concentrations were used in order to prepare nanofluid. The thermal conductivity and viscosity values needed for the calculations were obtained by measuring separately. For the experiments, the microchannels made by both the different materials (Stainless steel, PEEK) and the different inner diameter (400, 750, 1000 μm) were tested for the different surface temperatures (283, 298, 313 K). In the tests, the nanofluids had the different inlet temperature (323–333 K), the volume flow rates (20, 35, 50 mL min−1) and the concentrations. Heat transfer rate, Nusselt number, pressure drop and friction factor results were calculated. The optimum conditions were determined by using Taguchi approach. The thermal performance and the pressure drop of the fluids were compared. The results showed that the best thermal performance was obtained for Al2O3 nanofluid with 1.0% vol. concentration. A heat transfer enhancement of 15.3% was achieved using nanofluid instead of deionized water as the base fluid. Moreover, it has been seen no considerable pressure drop.

Characteristics of combined electroosmotic flow and pressure-driven flow in microchannels with complex-wavy surfaces

A numerical investigation is performed into the flow characteristics of various electrokinetic and pressure-driven flows within microchannels with complex-wavy surfaces. Four different flows are considered, including (1) pure electroosmotic flow; (2) pure pressure-driven flow; (3) combined electroosmotic/pressure-driven flow with a favorable pressure gradient; and (4) combined electroosmotic/pressure-driven flow with an adverse pressure gradient. The effects of the wavy surface geometry parameters and the ratio of the electroosmotic flow velocity to the pressure-driven flow velocity on the fluid flow characteristics are examined. The results show that while flow recirculations are induced by pure pressure-driven flow, recirculation structures are not formed in pure electroosmotic flow. In addition, it is shown that electrokinetically induced velocity is more sensitive than pressure-induced velocity to the waveform geometry. For combined electroosmotic/pressure-driven flow with a favorable pressure gradient, the momentum of the combined flow is sufficient to prevent the formation of flow recirculations. However, for combined electroosmotic/pressure-driven flow with an adverse pressure gradient, flow recirculations are induced near the wave crest when the ratio of the electroosmotic flow velocity to the pressure-driven flow velocity falls below a certain threshold value. It is observed that the recirculation structures are longer and thinner than those that are generated near the wave trough under pure pressure-driven flow conditions. The heat transfer characteristics for various flow scenarios are also investigated in the complex-wavy surface microchannel with constant surface temperature conditions by considering the Joule heating effect. The results show that the thermal entrance length significantly depends on the ratio of the electroosmotic flow velocity to the pressure-driven flow velocity. The longest entrance length is presented in the flow scenario of the favorable pressure gradient combined flow. In a thermally fully developed region, the heat transfer performance is dependent on the magnitude of the Joule heating and the geometry structure and is independent of flow scenarios.

Approximate solutions to the Stefan problem with internal heat generation

Using a quasi-static approach valid for Stefan numbers less than one, we derive approximate equations governing the movement of a phase change front for materials which generate internal heat. These models are applied for both constant surface temperature and constant surface heat flux boundary conditions, in cylindrical, spherical, plane wall and semi-infinite geometries. Exact solutions with the constant surface temperature condition are obtained for the steady-state solidification thickness using the cylinder, sphere, and plane wall geometries which show that the thickness depends on the inverse square root of the internal heat generation. Under constant surface heat flux conditions, closed form equations can be obtained for the three geometries. In the case of the semi-infinite wall, we show that for constant temperature and constant heat flux out of the wall conditions, the solidification layer grows then remelts.

Solidification inside a sphere—an experimental study

This paper presents the investigations of the solidification of an n-hexadecane inside a spherical enclosure. The effect of solidification process was investigated for three different constant surface temperature conditions (13 °C, 8 °C and 3 °C) and for three different initial superheats of n-hexadecane (8 °C, 2 °C and 0 °C). It was observed that the solidification phase front propagates uniformly inwards towards the centre of the sphere. The concentricity of the solid–liquid phase front deteriorates as time progresses due to shrinkages causing formation of voids inside the sphere. A lower constant surface temperature results in a larger solidified mass fraction. The effect of the initial liquid superheats of the PCM on the solidification is insignificant.

Discharge mode for encapsulated PCMs in storage tanks

This paper is aimed at analysing the behaviour of encapsulated salt hydrates, used as latent energy storage in a heat transfer system of a domestic hot water tank. The salt is a eutectic mixture of hydrate nitrates of ammonium and magnesium, with low melting temperature, already tested for latent heat storage in domestic applications. In the discharge mode, cold water enters the tank and flows on the encapsulated melted PCM, which is cooled and solidified. In the initial condition the PCM is at its melting temperature. Suddenly its external surface is cooled to a constant temperature T 0; the duration of the solidification represents the time in which the latent heat is released to water. The discharge process of the phase change material (PCM) is analyzed analytically and its effectiveness is assessed, for constant surface temperature conditions, in three different geometrical configurations, i.e. considering the PCM encapsulated in slab, cylindrical or spherical polyethylene containers. The focus is on a model of the moving boundary within the phase-change material during the discharging mode, and the duration of the phenomenon. Results shown include transient position of the moving surface, temperature distribution, amount of solid PCM, energy released, and duration of complete solidification. The influence of the geometry and the Jacob number on the ending time of solidification is investigated. Among different geometrical configurations of the PCM, it is found that the shortest time for complete solidification is matched for small spherical capsules, with high Jacob numbers and thermal conductivity.

Numerical simulation of non-linear heat conduction subjected to a laser source: the effects of variable thermal properties

This paper presents a numerical study of the temperature distribution in a body subjected to a spatially exponential decaying laser source. The governing heat conduction equation, the boundary conditions and the initial condition are presented in a dimensionless form as a function of a group of dimensionless parameters, namely, χ, τ, Bi, w, c 1, c 2 and c 3. Three different sets of thermal boundary conditions imposed at the “far” boundary, including convection, insulated and constant surface temperature conditions are investigated in the present study. The governing equation is discretized using a control volume approach, with a variable grid to increase the resolution of the domain near the boundary where the laser heat source is applied. The effects of the different parameters and the temperature dependent thermal properties are studied in detail. The calculated results are compared with previous analytical studies for constant thermal properties obtained for both semi-infinite and finite domains. Finally, the present numerical solutions are compared to existing experimental data.

Laser assisted vascular welding with real time temperature control.

Previous studies in laser assisted vascular welding have been limited by the lack of a reliable end point for tissue fusion. As a means of improving the reproductibility of laser assisted repairs, a system incorporating real time temperature monitoring and closed loop feedback was used. The system consisted of a direct view infrared thermometer for monitoring the laser heated spot, a 1.9 microns diode laser, and a microprocessor for data acquisition and feedback control of the laser power to maintain a constant tissue temperature. Rat aortas were welded under constant surface temperature conditions. In vivo temperature stability of +/- 2 degrees C was achieved over a temperature range of 70-90 degrees C pertinent to welding small vessels. When welds were completed using the feedback system to maintain the tissue temperature at 80 degrees C, the acute success rate was 100% and the burst pressure was 290 +/- 70 mmHg. These studies demonstrate that the use of real time monitoring and feedback control results in improved consistency for vascular tissue welding.

Thermal conductivity enhancement for phase change storage media

Thermal energy storage can be used as either a heat source or heat sink for spacecraft energy conversion and heat rejection systems. One major issue that needed to be addressed is that most phase-change materials (PCMs) with high energy storage density have an unacceptably low thermal conductivity. Two enhancement techniques are proposed in this paper. The baseline case utilizes LiH encapsulated in a thin SS304 container. The first enhancement technique uses LiH encapsulated in smaller containers of various shapes contained in a bigger cylindrical container filled with Li. The second enhancement technique focuses on a metal/phase-change material (M/PCM) composite. Specifically, 5% Ni is added uniformly to LiH so that the mixture has the same volume as the baseline storage unit. The enhancement techniques are analyzed numerically and their effectiveness is assessed for both constant surface heat flux and constant surface temperature conditions. Results shown include the amount of PCM melted as a function of time and the maximum temperature within the storage units. The M/PCM composite gives the best enhancement in thermal conductivity.

Heat Transfer and Friction Loss in Laminar Radial Flows through Rotating Annular Disks

Heat transfer and pressure drop performance are experimentally studied for laminar radial flow through a stack of corotating annular disks. The disk surfaces are heated by condensing steam to create constant surface temperature condition. The traditionally defined friction factor is modified to include the effect of centrifugal force induced by the rotation of the heat transfer surface on core pressure drop. Empirical equations are derived for the heat transfer and friction factors at zero rotational speed. Test results are obtained for various rotational speeds. It is disclosed that (1) The transition in the radial flow through rotating parallel disk passages occurs at the Reynolds number (based on the hydraulic diameter of the flow passage) of 3000 at which stall propagation occurs in the rotor. (2) In the laminar flow regime, its heat transfer performance at zero rotational speed is superior to forced convection in the triangular, square, annular, rectangular and parallel-plane geometries. (3) The effects of disk surface rotation are twofold: a significant augmentation in heat transfer accompanied by a very substantial reduction in friction loss. (4) These rotational effects decrease with an increase in the fluid flow rate until the transition Reynolds number where the effects of centrifugal and Coriolis forces diminish is reached. (5) Heat transfer performance at low through flows is superior to that of high-performance surfaces for compact heat exchangers.