Showing papers in "Heat Transfer Engineering in 1988"

Review of various Types of Flow Maldistribution in Heat Exchangers

Abstract: The types of maldistributions and their causes are discussed. Where possible, means of-avoiding or curing the problem are given. While the performance loss is often small, there are associated mechanical problems that can be severe.

wall Superheat Excursions in the Boiling incipience of Dielectric Fluids

Abstract: Many of the candidate fluids for immersion cooling of microelectronic components possess both low surface tension and high gas solubility. As a consequence, ebullient heat transfer with such fluids is accompanied by nucleation anomalies and a frequently observed wall temperature overshoot. The difficulty in preventing this thermal excursion and in predicting its magnitude constrains the development of immersion cooling systems. This paper begins with a brief review of the mechanisms that may be responsible for delayed nucleation and examines the limited literature on incipience superheat excursions.

Forced convective heat transfer from LSI packages in an air-cooled wiring card array

Abstract: The resistance to heat flow from finned LSI packages to the cooling air flow has been studied by combining the physical models with the results of heat transfer experiments, flow visualization experiments, and pressure drop measurements. Although crude assumptions are employed in analytical modeling, the proposed method of prediction of heat transfer and pressure drop has proved useful to cooling system designers. A notable finding is the advantage obtained by placing the packages in a staggered arrangement on the card. This reduces the local air temperature rise by as much as 70% over the conventional in-line arrangement, where the fan power is given as a constraint.

Effect of variations in Streamwise Spacing and Length on convection from Surface-Mounted Rectangular Components

Abstract: The effect of variations in streamwise spacing and component length on convection from rectangular, surface-mounted components in a channel flow are reported. Component dimensions are the same order of magnitude as the channel wall-to-wall spacing. The channel Reynolds number, with air as the coolant, ranged from 670 to 3000. Flow visualization showed that under the above conditions the channel flow is transitional. The effect of variations in component streamwise spacing on the level of turbulence in the channel and on the interaction between the core of the channel flow and the recirculating flow in cavities between components is discussed. Pressure drop measurements show that the dominant loss mechanism is due to form drag caused by the components. Local heat transfer measurements are made using an interferometer. Analysis of the results shows that the overall heat transfer is properly correlated in terms of a flow Reynolds number based on the component length. At small-component Reynolds number, the o...

Comparison of Some Design Choices for Evaporative Cooler Cores

Abstract: Three different design options for closed-circuit evaporative cooler cores are compared by way of numerical examples obtained with the use of computer programs written for the three cases. It is concluded that bare-tube cooler performance can be considerably enhanced by the judicious use of fill material without resorting to the use of fins to increase the mass transfer area.

Using Metallic Coatings to Enhance Thermal Contact Conductance of Electronic Packages

Abstract: Recently a new thermomechanical model for coated contacts has been developed and shown to be quite accurate. After a brief overview of the theory, this paper concentrates on illustrating the utility of the new model by applying it to a common electronics packaging problem: heat transfer across an aluminum joint. Several soft metallic coatings are considered, and the thermomechanical model is used to predict the improvement in the contact conductance over that for a bare aluminum-to-aluminum joint. For each coating material, heat transfer performance is presented as a function of the coating thickness, the surface roughness, and the applied pressure. Finally, a parameter is proposed that allows candidate coating materials to be ranked.

Temperature Prediction on Substrates and integrated Circuit Chips

Abstract: This work deals with application of semianalytical methods for evaluation of temperature distribution on substrates and integrated circuit chips. This approach is based on a method proposed by Hein and Lenzi in 1969 which is a combination of Fourier transform. Green's function, and surface-element methods. The application of the method has evolved from a model that predicts the steady-state temperature on one-layer structures with lead connectors (modeled as lumped thermal resistances) and planar-discrete sources to a model that includes the effects of multiple layers and anisotropic thermal conductivity. Further generalization of the method to three new cases is presented. The first includes the transient thermal behavior in the one-layer structures with planar-discrete sources and anisotropic conductivity. The second deals with the steady-periodic behavior of two-layer structures with planar-discrete-periodic sources and anisotropic conductivity. The third case solves for the steady-state temperature in...

Departure from Natural Convection (DNC) in Low-Temperature Boiling Heat Transfer Encountered in Cooling Microelectronic LSI Devices

Abstract: Recent experiments with low-temperature boiling heat transfer have shown that the determination of the transition point from natural convection to nucleate boiling is important in cooling microelectronic components. This paper discusses the nature of this transition, introduced here as the departure from natural convection (DNC). It is shown that the DNC, which is usually accompanied by excessive temperature overshoot, is just as important a parameter as is the well-known departure from nucleate boiling, (DNB), and that more work is required to understand the behavior of DNC. Some novel surface treatments applied to large-scale integrated (LSI) devices are also described that remove these limiting conditions of DNC and DNB.

Thermal, hydraulic, and corrosion aspects of PWR steam generator problems

Abstract: Two types of PWR steam generators are in use: recirculating steam generators (RSGs) and once-through steam generators (OTSGs). Most of the units and vertical and this review will be limited to vertical units. Some of these steam generators have operated with a minimum of problems while other steam generator designs have experienced a variety of corrosion and mechanically induced problems. The discussion will focus more on the designs that have experienced problems and the effort expended to correct them. Corrosion problems include denting, wastage, intergranular attack, stress corrosion cracking, corrosion fatigue, pitting on the outside surfaces of the tubes, and stress corrosion cracking the inner surfaces of the tubes. Mechanical concerns have included water hammer, thermal stratification in feed-water pipes, fretting and wear of the tubes caused by excessive tube vibration, and erosion-corrosion. These problems have caused unscheduled outages and expensive repairs.

Forced Draft Cooling Tower Performance with Diesel Power Stations

Abstract: A prototype cooling tower was used to explore the potential of using cooling towers compared with radiator cooling systems with 3 MW diesel engines. The working parameters were the water mass flow rate, water inlet temperature, air mass flow rate, and humidity ratio. The water mass flow rate was relatively the most effective. Three methods of calculation were used to evaluate performance—namely, heat and mass balance, psychrometric chart, and the heat and mass transfer method. The first was the best in comparison with experiments. The economic analysis of both the cooling tower and radiator systems showed that it would be more economical in the long run to use cooling towers for diesel engines.

Analytical Development and Computer Modeling of a Bellows-Type Heat Pipe for the Cooling of Electronic Components

Abstract: Presented here is a parametic study of the relationships that govern the operation of a bellows heat pipe for use in the cooling of electronic components. The defining equations are combined with the proper similarity relations to develop a mathematical model that can be used to describe the operational characteristics and performance limitations of such a heat pipe. The paper starts with a review of the recent experimental work done in the area of bellows heat pipes, formulates the governing equations, and proceeds through the development of a computer model. Preliminary testing of the computer model was carried out using experimental data obtained from previous investigations, to ensure that it accurately reflected the heat pipe performance. Once the model had been verified, it was used to develop an optimum design for a small bellows heat pipe.

Reboilers with Enhanced Boiling Tubes

Abstract: The fundamental and practical considerations involved in applying enhanced boiling tubes to reboilers and evaporators in the chemical and petrochemical industries are surveyed. Typical enhanced boiling performances, heat exchanger configurations, and process applications are also discussed.

A Microcomputer-Based Data Acquisition System and Software for Thermal Studies

Abstract: A data acquisition system used for temperature and mass concentration measurements is described. Data logging devices interfaced with a microcomputer are shown to offer a large data collection capacity suitable for transient measurements. Additional hardware required to interface conductivity probes to a microcomputer via a commercially available data logger is illustrated. Practical details on software to facilitate the data communication is described, and software for data reduction written in the C programming language is also provided.

Evaluation of Heat Pipes for Conduction-Cooled Level II Avionic Packages

Abstract: The application of heat pipes for the thermal management of conduction-cooled avionic packages using VHSIC technology (l-W to 3-W chips) is addressed. Test results are used to show the increase in the allowable power of the conduction-cooled avionic package obtained with heat pipes. Thermal performance was satisfactory for both horizontal and vertical attitudes. Performance predictions with a finite-element model are used to provide an understanding of the constraints that limit thermal performance and package power. Two Hughes prototype heat pipes using methanol as the working fluid are evaluated for two package configurations: one for a pluggable 64-mm multichip module (MCM) dissipating 50 W and the other for single-chip modules (SCMs) surface-mounted on a printed wiring board (PWB). Both heat pipe designs are characterized relative to transport capacity, liquid blockage, and heat flux.

Heat Transfer Analysis of Digital Transmission Equipment with Horizontally Arranged Printed Circuit Boards

Abstract: Printed circuit boards (PCBs) may be arranged horizontally in many situations, as for example in some digital transmission equipment being used in Brazil Such pieces of equipment, known as slim racks, have a high aspect ratio (height/width) and are assembled side by side and back to back The present work deals with the steady-state thermal dissipation in the so-called thermal unit of the slim rack made of the PCB, the magnetic shield above it and the bounding walls constituting an enclosure The heat transfer in each thermal unit was investigated by assuming isothermal enclosure surfaces with uniform radiosities Once the thermal paths are identified, an electric circuit analogy is employed to obtain the overall thermal resistance network Most of the thermal resistances are temperature-dependent, so the associated nonlinear algebraic equations can be solved only by an iterative method An experimental investigation was carried out in a module of the slim rack with simplified boundary conditions in orde

Effect of Uncertainties on the Performance of a Feed-Effluent Heat Exchanger System

Abstract: A computer program is developed to predict the performance of heat exchanger systems with or without feedback. The program is used to analyze the effect of uncertainties on the performance of a feed-effluent heat exchanger system. By analyzing the sensitivity of the outcome variables to the uncertainties in the inputs, the most critical component of the system is identified. Applicability of the effective measures to limit the uncertainties and thereby improve the performance of the system is also discussed.

The accident at Chernobyl Unit 4 in the Ukraine - April 1986

Abstract: On April 26, 1986, the worst accident in the history of commercial nuclear power generation occurred at the Chernobyl Nuclear Power Station some 60 miles north of Kiev in the Ukraine. This paper describes the sequence of events that occurred and the consequences of the accident. There was extensive damage to the Unit 4 reactor and the building that housed it. Some 31 people died as a result of the accident, either directly or as a result of receiving lethal radiation doses. A significant release of fission products occurred, contaminating land around the station and requiring the evacuation of 135,000 people from their homes. The radioactive cloud generated over several days was carried by winds to all parts of Europe, where there was a varying degree of public concern. The contamination resulted in restrictions on the consumption of meat and vegetables. The latent health effects are, however, unlikely to be statistically significant when viewed against the normal mortality rate over the next 40 years. Although there were gross violations by the operators, the primary cause of the accident can be attributed to inherent design shortcomings in the RBMK reactor type, which is unique to the Soviet Union. The Russians havemore » said that the accident is not possible in any commercial reactor operational outside the USSR.« less

Practical Application of the Corresponding States Principle in Heat Transfer Engineering

Abstract: Thermodynamic generalization methods based on reduced pressures proposed in the 1960s are reviewed and updated to reflect the current state of the art. These corresponding states principle (CSP) generalization techniques provide the fluid flow system engineer and the heat exchanger designer with an additional tool that is simple, effective, and above all, more reliable, particularly in evaporator and condenser design practice, than current conventional semiempirical correlations.

Boiling Two Hydrocarbon/ water Systems at Atmospheric Pressure

Abstract: Work previously reported on boiling the nonane/water system over a small electrically heated aluminum tube bundle is extended to the heptane/water system. For this system it proved possible to produce vapors with a wide range of compositions. For similar tube wall temperatures boiling was much enhanced compared to boiling pure water. The effect on heat transfer coefficient of individual tube rows of change in the boiler liquid composition and in heat flux is reported and discussed. The nonane/water system was boiled with the tube bundle in square pitch and also rotated-square pitch configurations. The film of water on the boiling surfaces above the interface was more stable with the tubes in the square pitch configuration. This proved to have a marked effect on the relation between overall boiling heat transfer coefficient and the proportion of nonane in the boiler. The paper concludes with prediction of the boiling behavior of other immiscible aqueous binaries and discusses the difficulties inherent in a...