L. F. A. Azevedo

Observed Flow Reversals and Measured-Predicted Nusselt Numbers for Natural Convection in a One-Sided Heated Vertical Channel

A three-part study encompassing both experiment and analysis has been performed for natural convection in an open-ended vertical channel. One of the principal walls of the channel—the heated wall—was maintained at a uniform temperature, while the other principal wall was unheated. The experiments, which included flow visualization and Nusselt number measurements, were carried out with water in the channel and in the ambient which surrounds the channel. At Rayleigh numbers which exceeded a threshold value, the visualization revealed a pocket of recirculating flow situated adjacent to the unheated wall in the upper part of the channel. The recirculation was fed by fluid drawn into the top of the channel, adjacent to the unheated wall. Average Nusselt numbers for the heated wall were measured over a three orders of magnitude range of a single correlating parameter, which includes the Rayleigh number and the ratio of the channel length to the interwall spacing. The Nusselt numbers were found to be unaffected by the presence of the recirculation zone. Numerical solutions obtained via a parabolic finite difference scheme yielded Nusselt numbers in good agreement with those of experiment. The numerical results covered the Prandtl number range from 0.7 to 10.

Vertical-channel natural convection spanning between the fully-developed limit and the single-plate boundary-layer limit

The effect of interplate spacing on natural convection in an open-ended vertical channel bounded by an isothermal and an unheated wall was studied both experimentally and computationally. The investigation encompassed the full range of operating conditions of the channel, i.e. from the limit of the fully-developed channel flow to the limit of the single vertical plate. Overall, a 50-fold variation in the spacing between the channel walls was investigated, and the vertical plate was employed in order to achieve the limit of infinite spacing. The experiments were performed in water (Pr ≊ 5) with the aforementioned parametric variation of the interplate spacing and for an order of magnitude range of the wall-to-ambient temperature difference. The numerical solutions were carried out for the experimentally investigated operating conditions and took account of both natural convection in the channel and conduction in the wall. It was found that the flat plate heat transfer does not form an upper bound for the channel heat transfer. The channel heat transfer is particularly sensitive to changes in interplate spacing for narrow channels and at small temperature differences. The results for all operating conditions were brought tightly together in terms of the groups Nus and (S/H)Ras, and a highly accurate correlation encompassing eight decades of (S/H)Ras, was developed. Excellent agreement between the experimental results and the computational predictions was obtained.

A Critical Review of the Modeling of Wax Deposition Mechanisms

Abstract Deposition of high molecular weight paraffins on the inner wall of subsea production and transportation pipelines continues to be a critical operational problem faced by the petroleum industry. The accumulation of the deposited material on the inner wall of the lines may lead to increased pumping power, decreased flow rate or even to the total blockage of the line, with loss of production and capital investment. The present paper presents a critical review of the research effort devoted to the understanding and modeling of the basic deposition mechanisms that control the wax deposition processes. Molecular diffusion of paraffin has been identified as the dominant deposition mechanism. Most deposition models available in the literature make use of molecular diffusion as the sole mechanism for predicting spatial and temporal distributions of paraffin deposits. However, the study of the literature conducted in the present paper evealed that there is not enough experimental evidence to support this assumption. Other deposition mechanisms, such as Brownian diffusion of solid wax crystals may also play a role.

Pulsed air jet impingement heat transfer

Abstract Impingement heat transfer from a pulsing jet was studied experimentally. The pulsing jet was generated using a valve similar to a ball valve in design, which was rotated at a known frequency. Temporal average heat transfer characteristics of the jet were quantified using a thermographic infrared imaging technique, while the temporal characteristics of the flow were measured using hot-wire anemometry. Jet Reynolds numbers (based on time-average flow rate) in the range 4000–40,000 were investigated for pulsing jet frequencies ranging from the steady jet condition (zero frequency) to approximately 200 Hz. The experimental results indicate that, for the configuration studied, the heat transfer is degraded at all frequencies, generally in the range 0–20% relative to that of the steady jet at the same time-average flow rate. This is in spite of the significant increase in turbulence intensity for the pulsing jet. The power spectral density function for the pulsing jet reveals a dominant peak at the pulsing frequency, with secondary peaks believed to be due to natural frequency of the fluid delivery tubing downstream of the valve. The degradation in heat transfer for the pulsing jet is believed to be due to relatively small turbulent fluctuations superimposed on the instantaneous periodic flow.

Transient Pig Motion Through Gas and Liquid Pipelines

Simulation of the transient motion of pigs through liquid and gas pipelines is presented. The differential form of the mass and linear momentum equations for compressible liquid and gas flows were solved by a finite difference numerical technique. The fluid flow equations were combined with a linear momentum equation for the pig and a model for bypass flow through the pig. The pig/wall contact forces were simulated by a stick/slip model. The contact forces developed by disk pigs and the pipe wall were predicted by a postbuckling finite element analysis of the discs. Test cases representing typical pigging operations were studied using the numerical model developed. The fluid flow and pig behavior predicted by the model presented a reasonable behavior, and contributed for a better understanding of the pig dynamics through gas and liquid pipelines.

Experimental and numerical investigation of natural convection in convergent vertical channels

Abstract Natural convection heat transfer in convergent vertical channels has been investigated both by experiments and by numerical solutions of the conservation equations. The investigation encompassed half angles of convergence between 0 (parallel-walled channel) and 15°, and the working fluid was water ( Pr ∼5). The channel walls were maintained at a uniform temperature which exceeded the ambient temperature, thereby giving rise to an upflow through the channel. It was found that the Nusselt numbers for the convergent channels could be brought into very close agreement with those for the parallel-walled channel by employing correlation variables based on the maximum interwall spacing as the characteristic dimension. The experimentally determined and numerically predicted Nusselt numbers were in excellent agreement, both in magnitude and with regard to all observable trends.

Two-Fluid and Single-Fluid Natural Convection Heat Transfer in an Enclosure

Natural convection experiments were performed for an enclosure of square cross section containing either a single fluid or two immiscible fluids in a layered configuration. The two vertical walls of the cross section were respectively heated and cooled, while the two horizontal walls were adiabatic. The single-fluid experiments, performed with distilled water and with n -hexadecane paraffin (Pr = 5 and 39.2, respectively), yielded Nusselt numbers whose Rayleigh and Prandtl number dependences were perfectly correlated by a single dimensionless group. These single-fluid results were used as baseline information for the development of methods to predict the heat transfer in two-fluid layered systems. To test the utility of the predictive methods, experiments were carried out for water–hexadecane systems in which the position of the interface separating the liquids was varied parametrically. It was found that the experimentally determined, two-layer Nusselt numbers were in excellent agreement with the predicted values. The prediction methods are not limited to the particular fluids employed here, nor do they require additional experimental data for their application.

Wax Blockage Removal by Inductive Heating of Subsea Pipelines

Total blockage of subsea petroleum production lines due to wax deposition is a relevant problem for the industry. This problem has led to significant capital losses associated with the loss of production and the substitution of plugged lines. The present paper is a study of the feasibility of a remediation procedure aimed at helping the removal of wax plugs. In this procedure, the section of the oil line plugged with wax is inductively heated through an external coil positioned over the line at sea bed. The objective of the work is to estimate the level of electrical power required to soften the wax plug inside the line. To this end, a transient heat conduction model was employed to predict the temperature distribution in the line wall and the solid wax. This information was employed to estimate the basic dimensions of the heating coil section and the thermal insulation employed to minimize the heating losses to the cold sea water environment. A laboratory experimental study with a subsea pipeline section plugged with wax was conducted to verify the numerical model predictions and to test the performance of the inductive heating coil.

Resistive Force of Wax Deposits During Pigging Operations

One of the forces that act on a pig while in operation in a pipeline is that due to the layer of wax deposited on the pipeline wall. Therefore, if the motion of a pig or its cleaning efficiency are to be determined, this resistive force must be known as a function of the relevant parameters. The goal of this paper is to present a procedure for evaluating this force in situations of engineering interest. In essence, the procedure consists of the following steps: (i) determine the wax shear strength according to an experimental procedure developed for this purpose; (ii) from finite-element results, obtain the maximum stress in wax; (iii) with the foregoing information, calculate the pressure required for the pig to cause a maximum stress on wax equal to its shear strength. This pressure value is the threshold below which no wax removal can occur, according to the present simple approach. In this paper, each of these steps is described in detail and exemplified.

Heat (Mass) Transfer for Circular Jet Impingement on a Confined Disk With Annular Collection of the Spent Air

Heat (mass) transfer experiments have been performed for a single circular jet impinging perpendicular to a confined disk, with the spent air being collected in an annulus which surrounds the jet delivery tube. This configuration provides precise control of the surface area affected by the impinging jet and also assures complete collection of the spent air. During the course of the experiments, parametric variations were made of the dimensionless separation distance between the jet origin and the impingement disk, of the ratio of disk diameter to the jet diameter, and of the Reynolds number. It was found that the heat (mass) transfer coefficient at the impingement surface increased substantially with a decrease in the jet diameter. Furthermore, for the smaller diameter jet, there was an optimum separation distance at which a maximum value of the heat (mass) transfer coefficient was achieved. For a jet of larger diameter, the transfer coefficient decreased monotonically as the separation distance increased.