Purpose

This paper aims to examine the unsteady stagnation-point flow, heat and mass transfer of upper-convected Oldroyd-B nanofluid along a stretching sheet. The thermal conductivity is taken in a temperature-dependent fashion. With the aid of Cattaneo–Christov double-diffusion theory, relaxation-retardation double-diffusion model is advanced, which considers not only the effect of relaxation time but also the influence of retardation time. Convective heat transfer is not ignored. Additionally, experiments verify that with sodium carboxymethylcellulose (CMC) solutions as base fluid, not only the flow curve conforms to Oldroyd-B model but also thermal conductivity decreases linearly with the increase of temperature.

Design/methodology/approach

The suitable pseudo similarity transformations are adopted to address partial differential equations to ordinary differential equations, which are computed analytically through homotopy analysis method (HAM).

Findings

It is worth noting that the increase of stagnation-point parameter diminishes momentum loss, so that the velocity enlarges, which makes boundary layer thickness thinner. With the increase of thermal retardation time parameter, the nanofluid temperature rises that implies heat penetration depth boosts up and the additional time required for nanofluid to heat transfer to surrounding nanoparticles is less, which is similar to the effects of concentration retardation time parameter on concentration field.

Originality/value

This paper aims to explore the unsteady stagnation-point flow, heat and mass transfer of upper-convected Oldroyd-B nanofluid with variable thermal conductivity and relaxation-retardation double-diffusion model.

中文翻译：

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具有可变热导率和弛豫延迟双扩散模型的上对流Oldroyd-B纳米流体的非定常驻点流动

目的

本文旨在研究沿拉伸片的上部对流 Oldroyd-B 纳米流体的非定常驻点流动、传热和传质。以与温度相关的方式获取导热率。借助Cattaneo-Christov双扩散理论，提出了弛豫-延迟双扩散模型，既考虑了弛豫时间的影响，又考虑了延迟时间的影响。对流传热也不容忽视。此外，实验验证了以羧甲基纤维素钠（CMC）溶液为基液，不仅流动曲线符合Oldroyd-B模型，而且热导率随温度升高呈线性下降。

设计/方法/方法

采用合适的伪相似变换将偏微分方程转化为常微分方程，通过同伦分析方法（HAM）进行解析计算。

发现

值得注意的是，驻点参数的增加减小了动量损失，从而使速度增大，使边界层厚度变薄。随着热阻时间参数的增加，纳米流体温度升高意味着热穿透深度增加，纳米流体向周围纳米颗粒传热所需的额外时间更少，这类似于浓度延迟时间参数对浓度场的影响.

原创性/价值

本文旨在探索具有可变热导率和弛豫延迟双扩散模型的上对流Oldroyd-B纳米流体的非定常驻点流动、传热和传质。