Uniform Surface Temperature Research Articles

Recent Advances in Closed Loop Spray Cooling and its Application in Airborne Systems

In recent years, the problem of heat dissipation in airborne directed energy weapons has attracted considerable research interest. Spray cooling can be applied to cool airborne directed energy weapons, owing to its several advantages such as a large heat transfer coefficient, absence of boiling hysteresis and uniform surface temperature. To examine the potential of an airborne spray cooling system, the typical high heat flux dissipation methods were compared, and the state of the art research on spray cooling was reviewed. This review was focused on studies related to the spray cooling hydrodynamic mechanism, experimental studies of closed loop spray cooling, numerical simulation studies about spray cooling and the identification of the factors influencing spray cooling systems, and investigations related to the multiple nozzle spray cooling technology and heat transfer correlation predictions. Overall, there is a need for further research to investigate the failure phenomenon after the critical state, matching operation of the total system and microscopic characteristics of airborne specific parameters.

NUMERICAL INVESTIGATION OF DESIGN PARAMETERS EFFECTS ON PERFORMANCE OF COOLING SYSTEM DESIGNED FOR A LITHIUM ION CELL

A 3D numerical approach using the Finite Element Method (FEM) is applied to model the thermal behavior of multilayer 20Ah LiFePO4/Graphite cell and to design a cooling system. A three-dimensional multilayer cell model with heterogeneous thermal properties for the various cell layers is developed to study the effects of design parameters on cooling performance of mini-channel aluminum plates. As design parameters, effects of channel width, a number of channel passes, inlet mass flow rate, and heat transfer medium were considered. Using the optimized parameters, cooling performance of water-cooling, and air-cooling systems were compared. The results showed that the designed cooling system provided good cooling performance in controlling the temperature rise and uniformity. Inlet mass flow rate was the main influential parameter in controlling the cooling performance. The optimum number of channel passes was found to be seven passes. Channel width mainly controlled the pressure drop and had minor effects on temperature. At higher discharge current rates, the water-cooling system showed better cooling performance in dropping the maximum temperature and making uniform surface and inner temperature profile. Moreover, pressure drop, and power consumption rates become significantly lower for water cooling system.

Sensitivity of Atmospheric River Vapor Transport and Precipitation to Uniform Sea Surface Temperature Increases.

Filaments of intense vapor transport called atmospheric rivers (ARs) are responsible for the majority of poleward vapor transport in the midlatitudes. Despite their importance to the hydrologic cycle, there remain many unanswered questions about changes to ARs in a warming climate. In this study we perform a series of escalating uniform SST increases (+2, +4, and +6K, respectively) in the Community Atmosphere Model version 5 in an aquaplanet configuration to evaluate the thermodynamic and dynamical response of AR vapor content, transport, and precipitation to warming SSTs. We find that AR column integrated water vapor (IWV) is especially sensitive to SST and increases by 6.3–9.7% per degree warming despite decreasing relative humidity through much of the column. Further analysis provides a more nuanced view of AR IWV changes: Since SST warming is modest compared to that in the midtroposphere, computing fractional changes in IWV with respect to SST results in finding spuriously large increases. Meanwhile, results here show that AR IWV transport increases relatively uniformly with temperature and at consistently lower rates than IWV, as modulated by systematically decreasing low‐level wind speeds. Similarly, changes in AR precipitation are related to a compensatory relationship between enhanced near‐surface moisture and damped vertical motions.

Spectral method solutions of a variable aspect ratio duct flow in a uniform transverse magnetic field with two different thermal boundary conditions

Abstract A laminar electrically-conducting flow inside an electrically-insulated rectangular duct in a uniform transverse magnetic field with both uniform surface temperature and uniform heat flux boundary conditions are considered numerically. The problem with aspect ratios of the range from 1:10 to 10:1 and Hartmann number M up to 1000 is solved using a highly accurate technique which is spectral method. The flow variables are expanded in terms of linear combinations of Chebyshev polynomials chosen to satisfy the boundary conditions implicitly. The resulting equations are collocated using the Gauss points to produce a system of nonlinear algebraic equations which is solved iteratively using Gauss elimination. Convergence properties of the numerical method reveals that for aspect ratio of less than 4 to 1, the flow is well resolved with as small as 29 by 29 Chebyshev polynomials; however, as the aspect ratio increases to more than 4 to 1, the number of polynomials required for an adequate resolution can be as high as 49 by 49 polynomials. It is found when the magnetic field is turned on, the pressure drop, in general, increases with the field for different aspect ratios. However, the pressure drop increase will be slower near the aspect ratio of 10. Also, the heat transfer increases with the field for most of the cases, but for some cases the field will have adverse effect on the heat transfer, particularly, for constant surface temperature boundary condition at aspect ratio > 4 and M 4 and M

Characterizing Drying in the South American Monsoon Onset Season with the Moist Static Energy Budget

AbstractThe tropical atmospheric circulation and attendant rainfall exhibit seasonally dependent responses to increasing temperatures. Understanding changes in the South American monsoon system is of particular interest given the sensitivity of the southern Amazon rainforest to changes in dry season length. We utilize the latest Geophysical Fluid Dynamics Laboratory Atmospheric Model (GFDL AM4) to analyze the response of the South American monsoon to uniform sea surface temperature (SST) warming. SST warming is a poorly understood yet impactful component of greenhouse gas–induced climate change. Region-mean rainfall declines by 11%, and net precipitation (precipitation minus evaporation) declines by 40%, during the monsoon onset season (September–November), producing a more severe dry season. The column-integrated moist static energy (MSE) budget helps elucidate the physical mechanisms of the simulated drying. Based on the seasonal analysis, precipitation reductions tend to occur when 1) a convecting region’s climatological MSE export is dominated by horizontal rather than vertical advection, and 2) the horizontal MSE advection increases in the perturbed climate, impeding ascent. On a synoptic scale, the South American low-level jet strengthens and exports more moisture from the monsoon sector, exacerbating spring drying.

Mathematical Analysis of Heat Transfer in Peristaltic Transport through a Rough Nonuniform Inclined Channel

A mathematical study for investigating heat transfer for the peristaltic flow of a Newtonian fluid through a nonuniform inclined channel while considering surface roughness of the inner wall has been conducted. Special attention is focused to analyze the physical behaviour of various flow characteristics at different magnitudes of surface roughness parameter. The results obtained for temperature have been characterized by two boundary conditions: uniform surface temperature and uniform heat flux. Effect of inner wall surface roughness and angle of inclination on the heat transfer for the peristaltic flow through a nonuniform channel has been discussed in detail. The expression for pressure gradient, velocity, and the temperature was obtained. The effect of inner wall surface roughness and the angle of inclination, on pressure rise and temperature, has also been elaborated in detail. The problem formulation developed has been solved using the long wavelength and low Reynolds number assumption.

Experimental study and optimization of a combustion-based micro thermoelectric generator

Thermoelectric generator (TEG) has drawn extensive attention for the power supply in various portable electronic products. This work developed a prototype of a combustion based micro-TEG, composed of micro combustor with high uniform surface temperature and wide flame stability range, thermoelectric (TE) module and water cooling heat sink. The effects of input fuel energy, equivalence ratio and hydrogen addition on the output power are tested. An energy balance analysis is conducted based on the experiment results. The results show that the lowest thermal efficiency exceeds 79%, which confirms the high efficiency of the micro combustor. A maximum total efficiency of 3.34% is obtained with input fuel power of 109 W and load resistance matching the internal resistance of TE module. An analytical model of the micro-TEG was developed and validated. Three kinds of TE materials are employed and the simulation results are compared. The effects of length and number of the TE leg on the total efficiency under different input fuel conditions are analyzed. A maximum total efficiency of 4.22% is obtained with optimization of the TE module structure. The heat loss through the gap between TE legs reduces the maximum total efficiency for lower length of the TE leg. This work deeply explains the interaction between the micro combustor and TE module, which gives valuable guideline for the optimization of the micro-TEG.

Global Land Monsoon Precipitation Changes in CMIP6 Projections

AbstractChanges in global land monsoon (GLM) precipitation determine the local water resource, affecting two thirds of global population. The future changes in GLM summer precipitation and the sources of projection uncertainty under four scenarios are investigated using the Coupled Model Intercomparison Project Phase 6 (CMIP6) models. The GLM summer precipitation is projected to increase by 1.76 ± 1.57% (2.54 ± 2.22%), 1.33 ± 1.97% (3.52 ± 3.05%), 0.96 ± 2.04% (3.51 ± 4.97%), and 1.71 ± 2.38% (5.75 ± 5.92%) in the near (long) term under Shared Socioeconomic Pathway (SSP) 1–2.6, SSP2–4.5, SSP3–7.0, and SSP5–8.5, respectively. The enhancement is caused by thermodynamic responses due to increased moisture, which is partly offset by dynamic responses due to weakened circulation. The uncertainty in GLM precipitation projection is the largest in SSP5–8.5 long‐term projection. The uncertainty of submonsoon precipitation projections is larger than that in GLM precipitation. The uncertainty of monsoon precipitation projection arises from the circulation changes, which can be partly explained by model‐dependent response to uniform sea surface temperature warming.

Optimization of film hole row locations of a nozzle guide vane using network approach

Fluid–thermal network simulations have been performed to optimize the each film hole row location of a high-pressure guide vane. In the present work, the optimized row locations were identified to minimize the coolant flow with different number of film holes configuration subjected to almost uniform surface temperature distribution on the nozzle guide vane. FlownexR2017 software was used to identify the optimal row location of a typical film-cooled vane. The results show that it is a useful simulation tool to obtain reduced pressure drop of film hole row in a relatively short turnaround time.

Performance of capillary ceiling cooling panel on ceiling surface temperature and indoor thermal environment

A new experimental methodology is presented to show the effect of water supply temperature, mass flow rate and thermal load distribution on the radiant ceiling capacity and thermal comfort conditions. Computerized fluid dynamics simulated vertical temperatures and velocities profiles were validated by a comparison with experimental results and the difference was within 10%. Uniform surface temperature distribution was achieved in a 45.6 m3 test room installed with capillary ceiling radiant cooling panels by an increase in water temperature and air supply velocity. When the ventilation system was turned off, the mean ceiling surface temperature rose from 16.9 ± 0.4°C to 21.5 ± 0.3°C with a rise in the inlet water temperature to 20.1°C. The temperature difference between the head and ankle of an occupant was 2.0°C, which complies with the Chinese standard, GB/T 18049-2017. At a height of 1–1.5 m, the maximum temperature fluctuation was 2°C in the horizontal direction. When the ventilation system was turned on, with the air supply temperature and velocity at 19.8°C and 1.11 m s−1, the ceiling surface temperature was increased by 0.5°C. The indoor air temperature has a positive correlation with the air supply temperature and internal heat load but a negative correlation with air supply velocity.

Transient heat transfer in fibrous multi-scale composites: A semi-analytical model and its numerical validation

This article presents the development and validation of a semi-analytical (hybrid) model to describe transient heat conduction in a composite material reinforced with long unidirectional cylindrical inclusions. The development of the model relies on the existing analytical solution at the particle-scale, subject to a locally uniform but time-evolving surface temperature; this solution is used to calculate the local heat exchange with the matrix material. Following this, spatial discretization (using N nodes) at the macroscale leads to a system of Ordinary Differential Equations, yielding the matrix temperature and the surface temperature of the inclusions at each node. n0 is the number of the terms in the Bessel expansion computed without recourse to approximation. The intra-fiber thermal response is recovered analytically from the above. A fully numerical two-dimensional model of a unidirectional composite containing 1000 randomly placed fibers is developed on the OpenFOAM platform, using Gmesh for generation of the computational meshes. Comparison of the predictions of the proposed semi-analytical model with the results of several 100s of numerical simulations, spanning a large range of size ratios (macro-scale to fiber diameter) and conductivity ratios, shows excellent agreement between the semi-analytical model and the numerical results. The proposed model is portable and executable on mid-level workstations, requiring minutes of CPU time for cases in which a full numerical solution would require 10s of CPU hours. It therefore provides an attractive and accurate alternative in modeling transient heat transfer in multi-scale fibrous composites.

Structure and geometric dimension optimization of interlaced microchannel for heat transfer performance enhancement

A novel interlaced microchannel is designed, and two side walls of the microchannel are served as the main heat transfer surface. The thermal and hydraulic performance of the interlaced and parallel microchannels are numerically investigated using the full-size conjugate heat transfer model, and then are compared with the experimental results. The optimized geometric dimensions of the interlaced microchannel including the depth, width and spacing are obtained. The results show that the maximum surface temperature deviation between the simulation and experiment of the interlaced and parallel microchannel are only 4.8% and 4.7%, respectively. When the cold water flow rate is 600 ml/min, the Nusselt number of the interlaced microchannel is increased by 65.4% compared to the parallel microchannel, and the JF factor reaches 1.438. The uniform surface temperature of the interlaced microchannel is obtained with a maximum temperature difference of 40 °C, which is lower than that of 50 °C in the parallel microchannel. The thermal and hydraulic performance of the interlaced microchannel are improved with the increase of the depth and width of microchannel due to the heat transfer area in the first and second heat transfer directions are increased. However, a large amount of cold water in the central position of the microchannel can’t participate in the heat transfer because of its larger depth and width, so 1.5 mm and 0.5 mm are selected as the optimized depth and width, respectively. Also, it is found that the spacing of microchannel has little effect on the thermal and hydraulic performance. Considering to the compactness and strength, 0.5 mm is the optimized spacing for heat transfer performance enhancement.

Concurrent Removal of Heat Transfer and Mass Flow Rate Nonuniformities in Parallel Channels of Microchannel Heat Sink

Nonuniform performance of the parallel channels of the microchannel heat sink is one of the major limitations aggravating many undesired influences in both the single phase and the two phase heat transfer in microchannel heat sink. The nonuniform performance of the channels is preliminarily governed by the nonuniformity of the mass flow rate and heat flux transferred from the interface of parallel microchannels. The poor heat transfer (adiabatic/natural convection) boundary condition is primarily responsible for heat flux nonuniformity. In the current study a novel design of parallel microchannels heat sink is proposed, which is obtained by decreasing nonuniformity of mass flow rate and heat transfer existing between parallel channels of the microchannel heat sink. The nonuniformity of the mass flow rate is removed by the existing variable width approach and heat transfer nonuniformity among parallel channels is solved by the newly developed slanted microchannel provision. Developed design normalizes flow nonuniformity by 95.5% and heat transfer nonuniformity by 97.5% in comparison to the conventional design microchannel heat sink. The two major benefits offered by the proposed design are cooled (4.2 K lower than the conventional design) and uniform (3.1 K lesser than the conventional design) base surface temperature. It is also found that proposed work even facilitated in reduction of average base temperature (1.3 K lower than the conventional design). The maximum improvement in Nusselt number is 4.07%, also the proposed design extends benefits at off-design conditions even.

Convective Aggregation and Indices Examined from CERES Cloud Object Data

AbstractConvective aggregation is a self‐aggregation phenomenon appearing in idealized radiative‐convective equilibrium simulations under constant, uniform sea surface temperature (SST). To gain an understanding of observed convective aggregation or organization, three metrics, i.e., simple convective aggregation index (SCAI), modified SCAI (MCAI), and convective organization potential (COP), are evaluated with cloud object data from CERES. MCAI is related to object sizes through a modified inter‐object distance (IOD). It is found that large‐size object groups are less aggregated according to SCAI but more organized according to COP, compared to small‐size object groups. The opposite sensitivities to object‐group size can be explained by the dominant roles of the IOD in SCAI and the sum of object radii in COP as object‐group sizes increase. However, large‐size object groups are slightly more aggregated than small‐size ones according to MCAI. Both SCAI and MCAI increase with the number of cloud objects (N) in an object group but COP has a weak dependency on N. Further sorting by object‐group total area shows that sensitivity of MCAI to object‐group area agrees with that of SCAI for small‐area ranges but with that of COP for large‐area ranges, which is related to the weak sensitivity of the modified IOD to object‐group area, as compared to that of the original IOD. Finally, the three metrics show the similar contrasts between continental and oceanic convection and the same weak sensitivity to SST. The latter suggests that self‐aggregation is weaker at higher SSTs than at lower SSTs, in contrast to the findings of many simulations.

Non-Newtonian effect on natural convection flow over cylinder of elliptic cross section

The non-Newtonian effect in the boundary layer flow over a horizontal elliptical cylinder is investigated numerically. A modified power-law viscosity model is used to correlate the non-Newtonian characteristics of the fluid flow. For natural convection flows, the surface of the cylinder is maintained by the uniform surface temperature (UST) or the uniform heat flux (UHF) condition. The governing equations corresponding to the flow are first transformed into a dimensionless non-similar form using suitable transformations. The resulting equations are solved numerically by an efficient finite difference scheme. The numerical results are presented for the skin friction coefficient and the local Nusselt number with the eccentric angle for different values of the power-law index n. The local skin friction coefficient and the local Nusselt number are found to be higher and lower, respectively, for the shear thickening fluids (n > 1) than the other fluids (n ⩽ 1). The effects of different elliptical configurations on the average Nusselt number are also presented and discussed for both conditions of the surface temperature.

Adaptive Process Control for Uniform Laser Hardening of Complex Geometries Using Iterative Numerical Simulation

Abstract Laser surface hardening, when applied to complex geometries, poses a challenge in the terms of obtaining uniform hardness throughout the hardened area because of variable heat sink effects. In this work, an iterative numerical approach was used to estimate the required modulation in laser power to achieve a uniform surface temperature throughout the process zone. Firstly, a transient thermal model for the laser–material interaction was developed using the finite element method for a rectangular spot of 8 × 5 mm. The temperature-dependent material properties were used to bring in nonlinear effects in the analysis so as to predict the hardened zone dimensions more precisely. The numerical model was validated by carrying out laser hardening experiments using a 6-kW diode laser. The validated numerical model was used with an iterative technique aided by conditional looping to achieve a uniform surface temperature during the laser hardening of the complex geometry with a variable heat sink. The developed iterative approach can be effectively used on any geometry with a variable heat sink to obtain a constant surface temperature throughout the process zone.

Theoretical analysis of thermal entrance problem for blood flow: An extension of classical Graetz problem for Casson fluid model using generalized orthogonality relations

An analysis of classical Graetz problem is carried out for the case of fluid obeying the Casson constitutive equation. The considered model physically corresponds to the thermal entry flow of blood in a duct. The governing equation for the premeditated problem is investigated by employing the separation of variables approach in conjunction with the MATLAB built in package bvp4c for the calculation of the eigenvalues and related numerical solution of the eigenvalue problem. The solution is computed for the case of uniform surface temperature boundary condition for both flat and circular geometries. The axial diffusion and viscous dissipation effects on temperature field are also taken into account. The expressions of bulk mean temperature and Nusselt number are presented and discussed in terms of the main effect brought by the yield stress parameter, Peclet number and Brinkman number. It is found that both local and mean Nusselt numbers for blood enhance considerably with the increase of dimensionless plug radius and Brinkman number. In contrast, the thermal entrance length reduces with the rise of Peclet number. The results of present analysis have potential applications in development of nano fluidic, microfluidic and bio-medical devices used in haemodialysis and oxygenation.

Experimental study on a novel thermal storage refrigerant-heated radiator coupled with air source heat pump heating system

Air source heat pump (ASHP) is a sustainable and efficient technology for space heating, which has been widely used over the recent decades. In order to improve the thermal efficiency of ASHP heating system, a novel thermal storage refrigerant-heated radiator coupled with ASHP heating system is proposed. In this system, the high temperature refrigerant steam discharged from the compressor is sent to the radiator directly to heat the space through radiation and natural convection. Energy storage water is injected to the radiator to supply energy during defrosting process. A set of experiments have been conducted in a climate chamber to study the performance of the radiator and system. The results show that the radiator has less heat dissipation and uniform surface temperature. As for system performances, the coefficient of performance varies from 4.4 to 3.3 as the condensing temperature increases from 35.9 °C to 44.0 °C, and it varies from 4.2 to 1.8 as the outdoor air temperature decreases from 6.9 °C to −18.4 °C. The defrosting performance is also investigated, proving that the energy storage material can maintain a comfortable indoor air temperature during the defrosting process. Furthermore, a characteristic equation specific to the refrigerant-heated radiator is given to provide reference for future application.

Automated Laser Ablation of Inhomogeneous Metal Oxide Films to Manufacture Uniform Surface Temperature Profile Electrical Heating Elements

This paper presents automated laser ablation strategies to improve the temperature distribution across the surface of inhomogeneous Ni-Fe-Cr-NiO electrical heating elements during joule heating. A number of iterative closed-loop laser control algorithms have been developed and analyzed in order to assess their impact on the efficacy of the heating element, in terms of homogeneous temperature control, and on the implications for automated fabrication of inhomogeneous metal oxide films. Analysis shows that the use of the leading method, i.e., use of a temperature-dependent variable-power approach with memory of previous processes, showed a 68% reduction in the standard deviation of the temperature distribution of the heating element and a greater uniformity of temperature profile as compared to existing manual methods of processing.

Self-sustained movement of condensate drop on uniform temperature surface during Marangoni dropwise condensation

During the condensation of some binary vapor mixtures so-called ‘positive system’, in which the more volatile component has the higher surface tension (e.g., water–ethanol), Marangoni dropwise condensation occurs owing to the temperature-induced local concentration and surface tension instability. It was found that the spontaneous movement of condensate drops occurred from the low- to high-temperature side of the heat transfer surface due to the concentration and surface tension difference around the drops, which are induced by the bulk temperature gradient. In this study, it was newly discovered that the spontaneous movement of condensate drops could be ‘self-sustained’ even without the presence of a bulk temperature gradient on the heat transfer surface. By applying the neighboring temperature gradient region and uniform temperature region on the condensing surface, two different modes of drop behaviors were observed during the experiments: (1) For the condensate drops with notable initial velocities in the temperature gradient region, the spontaneous movement was sustained in the uniform temperature region; (2) for the condensate drops initially formed in the uniform temperature region, the spontaneous movement did not occur owing to the lack of initial velocity. The reason for self-sustained movement of condensate drops can thus be inferred to be the presence of a self-formed temperature gradient around the moving condensate drops.