Chris J. Kobus

An experimental investigation into forced, natural and combined forced and natural convective heat transfer from stationary isothermal circular disks

Experimental heat transfer data are presented and dimensionless correlations developed for forced, natural and combined assisting forced and natural convection for heated stationary isothermal circular disks over wide ranges of the Reynolds, Rayleigh and modified Reynolds numbers, respectively. Experiments with air were performed for a variety of disks ranging in diameter and thickness-to-diameter aspect ratio. The correlation for combined forced and natural convection was developed utilizing the concept of a modified Reynolds number which accounts for a buoyancy-induced velocity. Utilizing this concept, the experimental data and respective empirical correlations for all three convection modes can be collapsed and plotted on the same continuous curve.

An experimental investigation into natural convection heat transfer from horizontal isothermal circular disks

Abstract Experimental heat transfer data are presented and a dimensionless correlation developed for natural convection from heated horizontal stationary isothermal circular disks over a wide range of the Rayleigh numbers. Experiments with air were performed for a variety of disks of different diameters and thickness-to-diameter aspect ratios. The significant volume of data is consistent with the single set of data available in the archival literature, and is correlated with a classical Nusselt–Rayleigh correlation.

Modeling the local and average heat transfer coefficient for an isothermal vertical flat plate with assisting and opposing combined forced and natural convection

Abstract A theoretical model is formulated, utilizing an integral technique, to describe the thermal boundary layer development. Special case closed-form solutions are obtained for 0.72 ⩽ Pr ⩽ 10 to predict the local and average heat transfer coefficient for combined forced and natural convection from an isothermal vertical flat plate, for both assisting and opposing flows. No opposing flow closed-form solutions are known to exist in the current literature. Assisting and opposing flow experiments were performed to measure the average heat transfer coefficient with air for two flat plate heat transfer models of different lengths. The predictive capability of the present theoretical model was compared to this experimental data with excellent agreement. Excellent agreement is also found to exist with the experimental data and numerical solutions of other researchers.

A theoretical investigation into the optimal longitudinal profile of a horizontal pin fin of least material under the influence of pure forced and pure natural convection with a diameter-variable convective heat transfer coefficient

The increased importance of heat sinks in electronic cooling applications has resulted in a revived interest in extended surfaces, or fins. Also, space and cost constraints provide impetus for optimizing thermal performance for a given, or least, amount of material. The current research focuses on a pin fin design of least material, where the diameter of the pin fin varies axially to maintain the axial heat flux constant; thus all fin material is utilized equally. Although such fins have been studied in the past, the convective heat transfer coefficient was assumed to be constant, which is not entirely true since it is known to be a function of diameter for cylindrical bodies. The current research shows that an optimal fin based on a variable convective heat transfer coefficient yields a true optimal profile, and utilizes material better; that is, it uses a lower volume of material to achieve the same heat dissipation rate. This improvement in material utilization is show to be anywhere from approximately 3% to 14%.

Predicting the Influence of Compressibility and Thermal and Flow Distribution Asymmetry on the Frequency-Response Characteristics of Multitube Two-Phase Condensing Flow Systems

An equivalent single-tube model concept was extended to predict the frequency-response characteristics of multitube two-phase condensing flow systems, complete with the ability to predict the influence of compressibility and thermal and flow distribution asymmetry. The predictive capability of the equivalent single-tube model was verified experimentally with extensive data that encompassed a three-order-of-magnitude range of frequencies, and a wide range of operating parameters.

True fluid temperature reconstruction compensating for conduction error in the temperature measurement of steady fluid flows

There are three major types of errors that can cause a temperature probe to read a value different from the true fluid temperature into which it is immersed. One type is the transient temperature lag error in general purpose temperature sensors. Another is an error due to radiation heat transfer from walls at high temperatures. The other type is the error due to thermal energy conduction in lead wires and/or thermowell or other casing from a base on which the temperature probe is mounted. Although much study has been devoted to the former, very little exists on the latter. In fact, any prior research on this subject assumed that the entire sensor was completely immersed in the fluid, which in many cases is not true. For all of these types of errors, the cause is the misunderstanding that all physical sensor types only measure their own temperature, which may or may not be equal to the fluid temperature into which they are immersed. This article focuses on conduction error in temperature measurements and q...

Predicting the average heat transfer coefficient for an isothermal vertical circular disk with assisting and opposing combined forced and natural convection

Circular disks are an important geometry when considering electronic component cooling, such as the cooling of disktype resistors and power transistors, or other related applications, such as the use of commercially available disk-type thermistors [1] for temperature and air flow measurements. Empirical correlations exist in the literature [1] for combined forced and natural convection from vertical circular disks for assisting flows, but not opposing flows. Also, to the best knowledge of the authors, there is no theoretical model currently available for predicting the convective heat transfer coefficient for assisting and opposing flows. Therefore, a conversion scheme is developed in the present research that utilizes a previously developed theoretical model [2], which was successful in predicting the average convective heat transfer coefficient for vertical flat plates experiencing combined forced and natural convection, to predict heat transfer coefficients for circular disks. The conversion scheme involves the concept of an effective flat plate length for a circular disk. The following flat plate solutions [2] may be used to predict the average heat transfer coefficient for circular disks by using this effective length, L* :

Predicting the Onset of a Low-Frequency, Limit-Cycle Type of Oscillatory Flow Instability in Multitube Condensing Flow Systems

A means was developed for extending the predictive capability of the Equivalent Single-Tube Model (ESTM) to accurately predict the onset of a self-sustained oscillatory flow instability for a multitube condensing flow system. The model includes the effects of compressibility, subcooled liquid inertia, and thermal and flow distribution asymmetry. Previously, liquid inertia, a necessary mechanism for the instability, had not been modeled for a multitube system. Extensive experimental data was obtained for a two-tube system that verifies not only the predictive capability of the ESTM, but also its accuracy and its wide range of applicability.

Storytelling - A Most Powerful Tool in Shaping Lesson Plans in Engineering Education

Most engineering instructors are put into the classroom by virtue of simply holding a Ph.D. in their field of study. Very few have any formal training in education or more specifically in engineering education, which would include a knowledge of various learning styles and how to engage them. Even knowing learning styles, one of the tools that is almost not utilized at all in engineering education is weaving in anecdotal evidence (ie — storytelling) into what is typically a logic-only lesson plan. Research shows that integrating right-brain activities (emotional, imaginative) into left-brain lesson (logical) very much increased student retention of material. Examples are given for how this can be done in mechanical engineering lessons specifically.Copyright

An Investigation Into the Effect of Subcooled Liquid Inertia on Flow-Change-Induced Transient Flow Surges in Horizontal Condensing Flow Systems

The objective of this research is to investigate large-scale transient flow surges of the condensate leaving in-tube condensing flow systems because of perturbations in the inlet vapor flow rate, and the influence of the subcooled liquid inertia of the condensate on these transient responses. Small changes in the inlet vapor flow rate momentarily cause large transient flow surges in the outlet liquid flow rate. Condensate inertia is seen to destabilize the system into an underdamped behavior where the flow rate can overshoot the final steady-state position several times. A one-dimensional, two-fluid, distributed parameter system mean void fraction (SMVF) model of the time-dependent distribution of liquid and vapor within the two-phase region is developed for predicting these transient characteristics, which it is seen to do quite well, especially when consideration is given to the complex nature of the problem.