Search results for "THERMODYNAMICS"

Local Effects of Longitudinal Heat Conduction in Plate Heat Exchangers

2007

Abstract In a plate heat exchanger, heat transfer from the hot to the cold fluid is a multi-dimensional conjugate problem, in which longitudinal heat conduction (LHC) along the dividing walls often plays some role and can not be neglected. Large-scale , or end-to-end, LHC is always detrimental to the exchanger’s effectiveness. On the contrary, if significant non-uniformities exist in the distribution of either convective heat transfer coefficient, small-scale , or local, LHC may actually enhance the exchanger’s performance by improving the thermal coupling between high heat transfer spots located on the opposite sides of the dividing wall.

A Photographic study of subcooled flow boiling burnout at high heat flux and velocity

2007

Abstract The present paper reports the results of a visualization study of the burnout in subcooled flow boiling of water, with square cross section annular geometry (formed by a central heater rod contained in a duct characterized by a square cross section). The coolant velocity is in the range 3–10 m/s. High speed movies of flow pattern in subcooled flow boiling of water from the onset of nucleate boiling up to physical burnout of the heater are recorded. From video images (single frames taken with a stroboscope light and an exposure time of 1 μs), the following general behaviour of vapour bubbles was observed: when the rate of bubble generation is increasing, with bubbles growing in the …

Evaluation of vapor mass transfer in various membrane distillation configurations: an experimental study

2011

Vapor mass transfer phenomena in four different membrane distillation (MD) configurations were examined through a self-built laboratory scale experimen- tal apparatus: Air Gap MD, Sweeping Gas MD, Vacuum Sweeping Gas MD and Vacuum MD. Vapor fluxes were measured and compared with those predicted by various models, showing that MD performance under usual processing conditions is severely controlled by the permeate side resistance to mass transfer.

Thermal and Hydrodynamic Analysis of the Melting Process in the Cold Crucible Using 3D Modeling

2008

The Nukiyama Curve in Water Spray Cooling: its Derivation from Temperature-Time Histories and its Dependence on the Quantities that Characterize Drop…

2007

Abstract Heat transfer from hot aluminium walls to cold water sprays was investigated. The method used was the transient two-side symmetric cooling of a planar aluminium target, previously heated to temperatures of up to 750 K, by twin sprays issuing from full-cone swirl spray nozzles of various gauge. The target’s mid-plane temperature was recorded during the cooling transient by thin-foil K thermocouples and a high-frequency data acquisition system. In order to determine the wall temperature Tw, the wall heat flux q w ″ and the q w ″ - T w heat transfer (Nukiyama) curve, two different approaches were used: the first was based on the solution of an inverse heat conduction problem, the seco…

On natural convection in a single and two zone rectangular enclosure

1992

Abstract Convective heat transfer was investigated numerically for rectangular enclosures both undivided and divided in two zones by a vertical partition, and having opposite isothermal walls at different temperatures. The aspect ratio was varied from 0.1 to 16 and the Rayleigh number from 3.5 ∗ 10 3 to ∗ 10 7 (non-partitioned enclosures) and from 1.0 ∗ 10 5 to 1.6 ∗ 10 8 (partitioned enclosures). The thickness and conductivity of the partition were varied. The end wall thermal boundary conditions were adiabatic or LTP (Linear Temperature Profile). The continuity, momentum and energy equations for a 2-D laminar steady flow were solved under the Boussinesq approximation by using a finite-dif…

Numerical simulation of reciprocating turbulent flow in a plane channel

2009

Direct numerical simulation results were obtained for oscillatory flow with zero time mean (reciprocating flow) in a plane channel using a finite volume method, Crank-Nicolson time stepping and central approximation of the advection terms. A pressure gradient varying co-sinusoidally in time was imposed as the forcing term, and its frequency and amplitude were made to vary so as to span a range of regimes from purely laminar to fully turbulent. For the limiting cases of reciprocating laminar flow and steady-state turbulent flow, numerical results were validated against analytical solutions and classic experimental literature data, respectively. For general reciprocating flows, predictions we…

Heat exchangers and linear image processing theory

1989

Abstract This paper shows that the transient analysis of some heat exchangers can be derived easily with the linear equations of image processing theory. Partial differential equations of the cross-flow, parallelflow and rotary heat exchangers are considered together with the corresponding discrete models for linear image processing. Some numerical examples show that the nature of the heat and/or mass transfer problems is similar to those of image processing.

On the influence of gravitational and centrifugal buoyancy on laminar flow and heat transfer in curved pipes and coils

2015

Abstract The effects of gravitational and centrifugal buoyancy on laminar flow and heat transfer in curved and helical pipes were investigated by numerical simulation. Six dimensionless numbers characterizing the problem were identified, and an analysis was conducted on the possible combinations of signs of the gravitational and centrifugal buoyancy effects. Two distinct Richardson numbers were introduced in order to quantify the importance of the two types of buoyancy, and it was shown that, in the case of heating from the wall, a maximum realizable value of the centrifugal Richardson number exists which is a linear function of the curvature δ (ratio of pipe radius a to curvature radius c)…

Heat and work fluxes in thermoelectric coolers

2021

Abstract Thermodynamics considers heat and work as the observables of energy. Then, in a non-equilibrium process, the fluxes of heat, work and energy are related. The expressions for the heat and energy flux densities are well known; although several conventions have been adopted. The work flux density defined from the heat and energy flux densities can be very useful in describing the energy balance when chosen to emphasize observable quantities. This paper discusses the advantages of the use of the conduction heat flux density given by Fourier’s law and a work flux density defined from it. As a case study, heat and work fluxes are evaluated in the elements of a thermoelectric cooler using…