Residence Time Distribution in Tubular Joule Effect Heaters with and without Geometric Modifications

Abstract

In the food industry, heat treatment of highly viscous fluids in continuous processes is becoming more and more common, and the process should perform as a homogenous thermal treatment, in order to ensure quality and safety of the final product. To improve treatment homogeneity, geometric modifications can be used even in the laminar regime, to induce flow perturbation and mixing. The objectives of this work include: (i) Investigation of the residence time distribution (RTD) for industrial indirect Joule effect heaters (JEH), with smooth (ST) and modified (MT) tubes, (ii) Demonstration and quantification of the efficiency of the geometrical modifications, and (iii) Proposition of a single semi-empirical model including the flow regime (10 < Re < 2000) and tube diameters (18 and 23 mm). The results obtained confirm that the simple Dispersed Plug Flow (DPF) model is not adaptable to small Reynolds numbers. Further analysis demonstrates that certain geometrical modifications improve the treatment homogeneity by increasing the plug flow contribution and reducing the value of the reduced variance. These beneficial effects increase when the Reynolds number is increased, the nominal diameter is reduced, and modified tubes are used. The proposed model enables the prediction of the RTD in JEH with an accurate degree of confidence.

Influence of flow regime and thermal power on residence time distribution in tubular Joule Effect Heaters

“…The performance of heat exchangers depends on the flow pattern, a hydraulic accident can break and renew the boundary layer of fluid, and consequently, higher heat transfer performances may be expected. André et al (2007) studied the benefits of geometrical modifications in tubular JEH which reduces the dispersion under isothermal condition. Irreducible coupling between heat transfer and fluid mechanics results in a complex flow pattern along modified and smooth tubular JEH.…”

“…The residence time of an element of fluid is defined as the time elapsed from its entry into the system until it reaches the exit (Villermaux, 1993). RTD theory and classical models are reported in Appendix B (André et al, 2007). Table 1 displays the general characteristics of smooth tube (ST) and modified tubes (MT).…”

“…2) exhibit a classical shape for industrial tubular heat exchanger including 180°junctions between tubes (André et al, 2007). In our condition, Churchill's equation (Appendix A) is simplified and modelled by three parameters (n, a and b with n 1 = 1 and n 2 = 2) which are experimentally identified in laminar and turbulent regimes and displayed in Table 3.…”

“…The experimental evaluation of the residence time distribution with the use of a plug flow model with axial dispersion part exchanging mass with a stagnant region, has allowed the determination of an effective axial dispersion coefficient with two configurations in the case of pseudoplastic fluid and for Re < 300. André et al (2007) and Fillaudeau et al (2001) investigated the impact of processing conditions on RTD in industrial JEH with smooth and modified tubes. A geometrical modification was selected in agreement with food applications considering pressure drop increase, heat transfer coefficient increase, propensity to fouling and cleaning efficiency (Lefebvre, 1998).…”

Influence of Rheological Behavior of Purely Viscous Fluids on Analytical Residence Time Distribution in Straight Tubes

“…On the other hand, the inconsistency in exposure time affects both quality and safety of such products. For example, too long exposure can cause denaturation and inactivation of nutritionally important ingredients of processed foods and also fouling of processing equipment, whereas too short exposure decreases the lethality factor [2][3][4][5]. Inconsistency in exposure time is directly related to the flow profile especially of viscous fluids with yield stress.…”