Wilfried Roetzel

Temperature Dependence of Thermal Conductivity Enhancement for Nanofluids

Usual heat transfer fluids with suspended ultra fine particles of nanometer size are named as nanofluids, which have opened a new dimension in heat transfer processes. The recent investigations confirm the potential of nanofluids in enhancing heat transfer required for present age technology. The present investigation goes detailed into investigating the increase of thermal conductivity with temperature for nano fluids with water as base fluid and particles of Al 2 O 3 or CuO as suspension material. A temperature oscillation technique is utilized for the measurement of thermal diffusivity and thermal conductivity is calculated from it

Conceptions for heat transfer correlation of nanofluids

Abstract The nanofluid is a solid–liquid mixture in which metallic or nonmetallic nanoparticles are suspended. The suspended ultrafine particles change transport properties and heat transfer performance of the nanofluid, which exhibits a great potential in enhancing heat transfer. The mechanism of heat transfer enhancement of the nanofluid is investigated. Based on the assumption that the nanofluid behaves more like a fluid rather than a conventional solid–fluid mixture, this article proposes two different approaches for deriving heat transfer correlation of the nanofluid. The effects of transport properties of the nanofluid and thermal dispersion are included.

Pool boiling characteristics of nano-fluids

Abstract Common fluids with particles of the order of nanometers in size are termed as ‘nano-fluids’ which have created considerable interest in recent times for their improved heat transfer capabilities. With very small volume fraction of such particles the thermal conductivity and convective heat transfer capability of these suspensions are significantly enhanced without the problems encountered in common slurries such as clogging, erosion, sedimentation and increase in pressure drop. This naturally brings out the question whether such fluids can be used for two phase applications or in other words phase change in such suspensions will be assistant or detrimental to the process of heat transfer. The present paper investigates into this question through experimental study of pool boiling in water–Al 2 O 3 nano-fluids. The results indicate that the nano-particles have pronounced and significant influence on the boiling process deteriorating the boiling characteristics of the fluid. It has been observed that with increasing particle concentration, the degradation in boiling performance takes place which increases the heating surface temperature. This indicates that the role of transient conduction in pool boiling is overshadowed by some other effect. Since the particles under consideration are one to two orders of magnitude smaller than the surface roughness it was concluded that the change of surface characteristics during boiling due to trapped particles on the surface is the cause for the shift of the boiling characteristics in the negative direction. The results serve as a guidance for the design of cooling systems with nano-fluids where an overheating may occur if saturation temperature is attained. It also indicates the possibility of such engineered fluids to be used in material processing or heat treatment applications where a higher pre-assigned surface temperature is required to be maintained without changing the fluid temperature.

Pool boiling of nano-fluids on horizontal narrow tubes

Abstract The search for new cooling medium does not limit itself to liquids alone. Liquid–solid suspensions have got a good promise in convective cooling applications. Suspension of common fluids with particles of the order of nanometers (typically 10–100 nm) in size are called ‘nano-fluids’ which have been found to enhance the heat transfer capability of the base fluid to a considerable extent. With very small volume fraction, such particles are capable of increasing the thermal conductivity and convective heat transfer significantly without the known problems encountered in common slurries such as clogging, erosion, sedimentation and increase in pressure drop. A recent study on pool boiling on a tube of large diameter (20 mm) shows that the nano-particles degrade the boiling performance with increasing particle concentration pushing up the wall superheat for a given heat flux. The present investigation focuses on an experimental study of pool boiling in water–Al2O3 nano-fluids on horizontal tubes of small diameter. Tubes of small diameter are of interest in efficient cooling applications such as those in electronic modules or LASER devices where miniaturisation is taking place at a rapid pace. However pool boiling of narrow horizontal tubes (4 and 6.5 mm diameter) is qualitatively different from the large diameter tubes due to difference in bubble sliding mechanism. It is found that at the range of narrow tubes the deterioration in performance in boiling is less compared to large industrial tubes which makes it less susceptible to local overheating in convective applications. Thus, the present study on boiling of nano-fluids can act as a guidance for the use of these engineered fluids in the above applications.

Experiment and analysis for non-Fourier conduction in materials with non-homogeneous inner structure

Abstract The proposition of hyperbolic conduction (also known as the second sound wave) for materials with non-homogeneous inner structure has run into a serious controversy in recent times. While one group of investigators has observed very strong evidence of hyperbolic nature of conduction in such materials and experimentally determined the corresponding relaxation times to be of the order of tens of seconds, the other group proclaims that their experiments do not show any such relaxation behaviour and the conventional Fourier law of conduction is good enough to describe conduction in them. This paper is an effort towards resolving this controversy. In the first place the experimental philosophies and techniques of both the groups have been thoroughly examined. It has been observed that determination of thermophysical properties independent of the relaxation time measurement is an inherent inconsistency in all these experiments. Additionally the assumptions regarding temperature input might have also played a role to arrive at diverging conclusions. Based on these observations an experimental method has been suggested in this study which uses temperature oscillation in semi infinite medium to determine the thermal diffusivity and the relaxation time simultaneously from a single experiment. Using this technique the wide range of experiments conducted reveal that there exists a definite hyperbolic effect in the “bulk” conduction behaviour of such materials although it is somewhat less in extent to those reported by investigators claiming existence of hyperbolic conduction. Also a wide range of experiments with variation of parameters such as packing material, its particle size, filling gas used and its pressure and temperature have been conducted. The data presented here for the wide range of parameters can be useful for further investigations and plausible explanation of “bulk conduction” in materials with non-homogeneous inner structure.

Transient response of the human limb to an external stimulust

Abstract The multidimensional transient temperature profiles of the arterial and venous blood flows and of the tissue within a limb are simulated with the bioheat equations developed by means of the heat transfer principle in porous media. The conjugated differential equations are solved numerically. Three different layer models are introduced to treat discrete distributions of the anatomical and thermal properties. Some examples are computed and discussed.

Dynamic behaviour of one-dimensional flow multistream heat exchangers and their networks

The dynamic behaviour of one-dimensional flow (cocurrent and countercurrent) multistream heat exchangers and their networks is modelled and simulated. The problems can be classified into two types: (1) dynamic responses to arbitrary temperature transients and to sudden flow rate transients from a uniform temperature initial condition or a steady-state condition, which yield a linear mathematical model; (2) dynamic responses to disturbances in thermal flow rates, heat transfer coefficients or flow distributions, which are non-linear problems and should be solved numerically. A linearized model is developed to solve the non-linear problems with small disturbances. The linear model and the linearized model for small disturbances are solved by means of Laplace transform and numerical inverse algorithm. Introducing four matching matrices, the general solution can be applied to various types of one-dimensional flow multistream heat exchangers such as shell-and-tube heat exchangers and plate heat exchangers as well as their networks. The time delays in connecting and bypass pipes are included in the models. The software TAIHE (transient analysis in heat exchangers) is further developed to include the present general solution and is applied to the simulation of fluid temperature responses of multistream heat exchangers. Examples are given to illustrate the procedures in detail.

A general solution for one-dimensional multistream heat exchangers and their networks

Abstract A mathematical model for predicting the steady-state thermal performance of one-dimensional (cocurrent and countercurrent) multistream heat exchangers and their networks is developed and is solved analytically for constant physical properties of streams. By introducing three matching matrices, the general solution can be applied to various types of one-dimensional multistream heat exchangers such as shell-and-tube heat exchangers, plate heat exchangers and plate–fin heat exchangers as well as their networks. The general solution is applied to the calculation and design of multistream heat exchangers. Examples are given to illustrate the procedures in detail. Based on this solution the superstructure model is developed for synthesis of heat exchanger networks.

Hyperbolic axial dispersion model for heat exchangers

Flow maldistribution in heat exchangers for steady-state and transient processes can be described by dispersion models. The traditional parabolic model and the proposed hyperbolic model which includes the parabolic model as a special case can be used for dispersive flux formulation. Instead of using the heuristic approach of parabolic or hyperbolic formulation, these models can be quantitatively derived from the axial temperature profiles of heat exchangers. In this paper both the models are derived for a shell-and-tube heat exchanger with pure maldistribution (without back mixing) in tube side flow and the plug flow on the shell side. The Mach number and the boundary condition which plays a key role in the hyperbolic dispersion have been derived and compared with previous investigation. It is observed that the hyperbolic model is the best suited one as it compares well with the actual calculations. This establishes the hyperbolic model and its boundary conditions.

Measurement of the heat transfer coefficient in plate heat exchangers using a temperature oscillation technique

Abstract Thermal parameters of plate type heat exchangers are experimentally evaluated using a temperature oscillation technique. A mathematical model with axial dispersion has been utilised to evaluate heat transfer coefficient and dispersion coefficients characterised by NTU and Peclet number, respectively. Special reference has been made to the deviation from plug flow due to the phase lag effect in a U-type plate heat exchanger. A mathematical model for correcting the thermal penetration effect in plate edgings and thick end plates has been presented. The experimental results obtained by using the temperature oscillation technique have been compared with those obtained by traditional steady-state experiment. A series of developments have been suggested to make the method more suitable for plate type heat exchangers.