Bruce T. Murray

Reactive wetting in metal–metal systems

Wetting and spreading in high temperature reactive metal-metal systems is of significant importance in many joining processes. An overview of reactive wetting is presented outlining the principal differences between inert and reactive wetting. New experimental evidence is presented that identifies an early time regime in reactive wetting in which spreading occurs without macroscopic morphological change of the solid-liquid interface. This regime precedes the heavily studied reactive wetting regime. Additional new experimental evidence is presented of kinetic roughening in a high temperature reactive system. Quantitative characterization of this roughening reveals similarities with room temperature systems. These new data provide evidence that supports the existence of several sequential time regimes in the reactive wetting process in which different physicochemical phenomena are dominant.

The effect of oscillatory shear flow on step bunching

Abstract During crystal growth, an imposed shear flow at the crystal–fluid interface can alter the conditions for the onset of morphological instability. In previous work, we studied the effect of time-independent shear flows and anisotropic interface kinetics on the morphological stability of a crystal growing from supersaturated solution. The model assumes that growth is by the motion of elementary steps, which is treated by a macroscopic anisotropic kinetic law; morphological instability corresponds to the bunching of elementary steps. Predictions from linear stability theory indicate that a solution flowing above a vicinal face of a crystal can either enhance or prevent the development of step bunches, depending on the direction of the steady shear flow in relation to the direction of step motion; this is also observed in experiments. Here we extend the linear stability analysis to include the effect of an oscillatory shear flow on the morphological stability of a crystal growing from solution and present results for a model system for a range of oscillatory shear rate amplitudes and frequencies both with and without a steady shear component. In the presence of a steady shear flow, modulation can either stabilize or destabilize the system, depending on the modulation amplitude and frequency. Numerical solutions of the linearized Navier–Stokes and diffusion equations and an approximate analytical treatment show that the flow oscillations weakens both the stabilization and destabilization induced by steady-state flow. This weakening comes from mixing of solution above the perturbed interface and a modification to the phase shift between the interface perturbation wave and the corresponding concentration and flow waves. Optimal values of modulation frequency and amplitude are found when the steady flow is destabilizing.

Numerical modeling approach to dynamic data center cooling

Data centers are facilities that house large numbers of computer servers that dissipate high power. Due to varying operational loads their efficient thermal management is a big challenge that needs to be addressed. Computational modeling using a CFD code is a very useful technique for studying the cooling requirements for different data center power loading and room configuration. Much of the existing numerical modeling research literature focuses on simulating the data center thermal environment with constant operating conditions. In reality, data centers have highly time dependent operating conditions i.e. fluctuations in server power and A/C flow rates. Recent computational studies have shown that time dependent fluctuations in server rack power can lead to rapid fluctuations in rack inlet temperatures. For optimal data center performance, the cold air supply should also increase or decrease with the rack power. In this paper, a detailed numerical study of data center thermal performance is presented with time dependent power and cooling air supply conditions. Results are presented for average rack inlet temperatures as a function of time for different case studies. Fluctuations in inlet temperatures are explained by evaluation of the temperature and flow fields in a basic data center configuration.

Effect of flow due to density change on eutectic growth

Abstract The Jackson–Hunt model of eutectic growth is extended to allow for different densities of the phases. The density differences give rise to fluid flow which is calculated from a series solution of the fluid flow equations in the Stokes flow approximation. The solute diffusion equation with flow terms is then solved numerically using an adaptive refinement and multigrid algorithm. The interface undercoolings and volume fractions are calculated as a function of spacing for tin–lead and iron–carbon eutectic alloys and for an aluminum–indium monotectic alloy. The numerical results are compared with various approximations based on the Jackson–Hunt analysis.

A Compact Thermal Model for Data Center Analysis using the Zonal Method

Modeling the thermal environment for data centers, including prediction of airflow and temperature distributions, is generally an extremely time-consuming process using full-scale CFD analysis. Reduced order models are necessary in order to provide real-time assessment of cooling requirements. The use of a coarse-grained zonal model is being investigated as a predictive tool, and this article details the development and implementation of a three-dimensional, pressurized zonal model. To construct and validate the zonal model, a basic data center configuration was analyzed using a finite volume software package. The calculated flow fields provide the spatial flow coefficients required in the zonal model, which is based on the power law method (PLM). A physically-based mapping between the controllable spatial mass flow rate and temperature distribution was obtained. Good agreement (within 10% average relative error) was obtained between the zonal model predictions and the CFD results. These preliminary results show promise that zonal models may yield an effective real-time thermal management design tool for data centers.

Cross Flow Heat Exchanger Modeling of Transient Temperature Input Conditions

The effectiveness of heat exchangers strongly influences the thermal performance of data center cooling systems. Rear door heat exchangers, in-row and overhead coolers, and fully contained water cooled cabinets are some examples of liquid and hybrid cooling systems used in data centers. Modeling the dynamic behavior of heat exchangers is important for the design of control strategies to improve energy efficiency. In this paper, an existing 2-D model of the transient temperature response of an unmixed-unmixed cross flow heat exchanger, of the type that is widely used in data center cooling equipment, is solved numerically. A detailed analysis of the transient response to a step, ramp, or exponential change in the hot fluid inlet temperature is conducted. The heat capacity rate ratio (E), thermal resistance (R), capacitance ratio (V), and number of transfer units are varied over a wide range to determine their influence on the heat exchanger dynamic thermal performance under different transient input conditions. In addition, the thermal response to the magnitude and time period of the transient input functions are evaluated. The modeling results are used to analyze specific data center cooling scenarios, and provide a means for predicting the transient behavior of heat exchangers used in data center cooling equipment.

Performance and Reliability Analysis of Hybrid Concentrating Photovoltaic/Thermal Collectors With Tree-Shaped Channel Nets' Cooling System

Excess temperatures on concentrating photovoltaic (PV) modules can lead to a decrease in electrical efficiency and irreversible structural damage. Therefore, designing an appropriate cooling system becomes necessary for the lifetime and performance of concentrating PV (CPV) modules. The basic design considerations for cooling systems include low and uniform cell temperatures, minimal pumping power, high PV efficiencies, and system reliability. In this paper, a 3-D multiphysics computational model for a hybrid concentrating photovoltaic/thermal (HCPV/T) water collector is developed. The collector consists of a solar concentrator, 40 silicon cells connected in series, and a multichannel liquid cooling system with heat-recovery capability. A conjugate heat transfer model is used, assuming laminar flow through either parallel or tree-shaped branching cooling channels. The temperature distributions within the PV cells are determined for different cooling strategies. Comparisons are made by considering the thermal and electrical performances, such as PV cell temperature, electrical efficiency, and outlet water temperature, between a system incorporating tree-shaped channel networks and one having straight parallel channel cooling arrays. For identical convective surface area and pumping pressures in both configurations, the tree-shaped branched channel cooling networks yield lower PV cell temperatures and more uniform temperature distributions within the PV cells. Additionally, a finite-element mechanical analysis is used to estimate the fatigue life of the PV modules based on the temperature profiles obtained from both cooling channel configurations under a specified pumping pressure. The model results predict that the fatigue life of the module with the branched channels is almost twice that of the module with straight channels.

Thermal Modeling and Life Prediction of Water-Cooled Hybrid Concentrating Photovoltaic/Thermal Collectors

In this paper, a multiphysics, finite element computational model for a hybrid concentrating photovoltaiv/thermal (CPV/T) water collector is developed. The collector consists of a solar concentrator, 18 single junction germanium cells connected in series, and a water channel cooling system with heat-recovery capability. The electrical characteristics of the entire module are obtained from an equivalent electrical model for a single solar cell. A detailed thermal and electrical model is developed to calculate the thermal and electrical characteristics of the collector at different water flow rates. These characteristics include the system temperature distribution, outlet water temperature and the thermal and electrical efficiencies. The model is used to study the effect of flow rate on the efficiencies. It is found that both efficiencies improve as the flow rate increases up to a point (0.03 m/s), and after that point remain at relatively constant levels. However, as the flow rate increases the outlet water temperature decreases, reducing the quality of the extracted thermal energy. In addition to the thermal and electrical modeling, finite element analysis is used to estimate the fatigue life of the module based on the different temperature profiles obtained from the thermal model at flow rates of 0.01 m/s and 0.03 m/s. Results show that for the higher flow rate, the outlet water temperature decreases, but the fatigue life improves. Based on the fatigue life model predictions, it is shown that the thickness of die attach layer must be inereased for high outlet temperature applications of the hybrid CPV/T collector. [.

Effect of Transient Boundary Conditions and Detailed Thermal Modeling of Data Center Rooms

With the ever increasing heat loads dissipated by computer hardware, the ability to accurately characterize the cooling requirements in data centers is becoming crucial. Computational fluid dynamics modeling has proven in many cases to be adequate in providing a general understanding of the thermal environment in data centers. However, almost all analyses of data centers to this day are conducted in steady state. The effect of changes in hardware configurations and cooling resources on server rack inlet temperatures and airflow through servers has not been adequately investigated. This paper introduces transient modeling of data centers with changing power dissipations and computer room air-conditioning (CRAC) airflow rates. Transient modeling proves advantageous in monitoring temperature fluctuations due to airflow changes, which in some cases leads to insufficient cooling. In addition, modeling server thermal mass is shown to be important for transient analysis, as it can lead to overshoots in temperatures. Another segment of this paper looks at the effect of introducing thermal characteristics curves into CRAC modeling on server inlet temperature. All the cases presented show that fixed CRAC supply temperature is a non-valid assumption.

Numerical modeling of data center with transient boundary conditions

Data centers are the facilities that house large number of computer servers that dissipate high power. Considering the dynamics of the data centers, their efficient thermal management is a big challenge that needs to be addressed. Computational analyses using a CFD code is very useful technique that helps the engineer to understand and solve the data center cooling problem. Several ongoing numerical modeling research efforts focus on simulating data center environment with constant boundary conditions. In reality, data centers have highly time dependent boundary conditions i.e. fluctuations in server powers and CRAC flow rate. In this paper, effect on time dependent boundary conditions on rack inlet temperatures will be discussed in detail with the transient modeling of data centers. Case studies will be presented to illustrate the transient fluctuations of rack inlet air temperature by the variation of rack power and CRAC flow rate.